* Distributed insertion to one random shard
* add some tests
* add some documentation
* Respect shards' weights
* fine locking
Co-authored-by: Ivan Lezhankin <ilezhankin@yandex-team.ru>
<details>
Stacktrace:
```
(gdb) bt
0 DB::appendBlock (from=..., to=...) at ../src/Storages/StorageBuffer.cpp:411
1 DB::BufferBlockOutputStream::insertIntoBuffer (this=<optimized out>, block=..., buffer=...) at ../src/Storages/StorageBuffer.cpp:541
2 0x000000000f2e9d5f in DB::BufferBlockOutputStream::write (this=<optimized out>, block=...) at ../src/Storages/StorageBuffer.cpp:508
3 0x000000000ec426c4 in DB::PushingToViewsBlockOutputStream::write (this=0x7f74660faa18, block=...) at ../src/DataStreams/PushingToViewsBlockOutputStream.cpp:160
4 0x000000000ec49633 in DB::AddingDefaultBlockOutputStream::write (this=0x7f74660f1b18, block=...) at ../src/DataStreams/AddingDefaultBlockOutputStream.cpp:10
5 0x000000000ec483ac in DB::SquashingBlockOutputStream::finalize (this=0x7f74660f1d18) at ../src/DataStreams/SquashingBlockOutputStream.cpp:30
6 0x000000000ec48429 in DB::SquashingBlockOutputStream::writeSuffix (this=0x7f74660f1d18) at ../src/DataStreams/SquashingBlockOutputStream.cpp:50
7 0x000000000ec43f8f in DB::PushingToViewsBlockOutputStream::writeSuffix (this=0x7f74660f8258) at ../src/DataStreams/PushingToViewsBlockOutputStream.cpp:280
8 0x000000000ec43f8f in DB::PushingToViewsBlockOutputStream::writeSuffix (this=0x7f74b7ddea18) at ../src/DataStreams/PushingToViewsBlockOutputStream.cpp:280
9 0x000000000f2e6748 in DB::StorageBuffer::writeBlockToDestination (this=<optimized out>, block=..., table=...) at ../src/Storages/StorageBuffer.cpp:820
10 0x000000000f2ea00b in DB::BufferBlockOutputStream::write (this=0x7f7574e11748, block=...) at ../src/Storages/StorageBuffer.cpp:469
11 0x000000000ec426c4 in DB::PushingToViewsBlockOutputStream::write (this=0x7f7574ed3658, block=...) at ../src/DataStreams/PushingToViewsBlockOutputStream.cpp:160
12 0x000000000ec49633 in DB::AddingDefaultBlockOutputStream::write (this=0x7f7574e84518, block=...) at ../src/DataStreams/AddingDefaultBlockOutputStream.cpp:10
13 0x000000000ec482f4 in DB::SquashingBlockOutputStream::write (this=0x7f7574e84718, block=...) at ../src/DataStreams/SquashingBlockOutputStream.cpp:17
14 0x000000000ebe8bce in DB::CountingBlockOutputStream::write (this=0x7f7574ed3720, block=...) at ../src/DataStreams/CountingBlockOutputStream.cpp:17
15 0x000000000f68e834 in DB::TCPHandler::receiveData (this=<optimized out>, scalar=<optimized out>) at ../src/Server/TCPHandler.cpp:1168
16 0x000000000f68737c in DB::TCPHandler::receivePacket (this=0x7f7574f17000) at ../src/Server/TCPHandler.cpp:918
17 0x000000000f688d2f in DB::TCPHandler::readDataNext (this=0x7f7574f17000, poll_interval=@0x7f6f1dff1f78: 10000000, receive_timeout=@0x7f6f1dff1f68: 300) at ../src/Server/TCPHandler.cpp:460
18 0x000000000f6878be in DB::TCPHandler::readData (this=0x7f7574f17000, connection_settings=...) at ../src/Server/TCPHandler.cpp:490
19 DB::TCPHandler::processInsertQuery (this=0x7f7574f17000, connection_settings=...) at ../src/Server/TCPHandler.cpp:519
20 0x000000000f680ab9 in DB::TCPHandler::runImpl (this=0x7f7574f17000) at ../src/Server/TCPHandler.cpp:268
21 0x000000000f68f297 in DB::TCPHandler::run (this=0x7f7574f17000) at ../src/Server/TCPHandler.cpp:1414
22 0x0000000011fb81cf in Poco::Net::TCPServerConnection::start (this=0x0) at ../contrib/poco/Net/src/TCPServerConnection.cpp:43
23 0x0000000011fb9be1 in Poco::Net::TCPServerDispatcher::run (this=0x7f752ab5fd00) at ../contrib/poco/Net/src/TCPServerDispatcher.cpp:112
24 0x00000000120e71c9 in Poco::PooledThread::run (this=0x7f747d3a4580) at ../contrib/poco/Foundation/src/ThreadPool.cpp:199
25 0x00000000120e315a in Poco::ThreadImpl::runnableEntry (pThread=<optimized out>) at ../contrib/poco/Foundation/src/Thread_POSIX.cpp:345
26 0x00007f760620aea7 in start_thread (arg=<optimized out>) at pthread_create.c:477
27 0x00007f760613aeaf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
(gdb) p to.data.__end_-to.data.__begin_
$17 = 10
(gdb) p to.data.__begin_[9].column.px
$19 = (const DB::IColumn *) 0x7f7328392720
(gdb) p to.data.__begin_[8].column.px
$20 = (const DB::IColumn *) 0x0
(gdb) p to.data.__begin_[7].column.px
$21 = (const DB::IColumn *) 0x7f746f33d360
```
Line numbers matched with this version -
f0e7cb16a7/src/Storages/StorageBuffer.cpp (L411)
</details>
