This is not that important, but because of packets overlaps the client
may not render the fatal error.
You will find TSan report in `details`.
<details>
```
WARNING: ThreadSanitizer: data race (pid=13384)
Write of size 8 at 0x7b24000ad818 by thread T47:
0 DB::WriteBuffer::next() obj-x86_64-linux-gnu/../src/IO/WriteBuffer.h:43:15 (clickhouse-tsan+0x16124d61)
1 DB::TCPHandler::sendProgress() obj-x86_64-linux-gnu/../src/Server/TCPHandler.cpp:1602:10 (clickhouse-tsan+0x16124d61)
2 DB::TCPHandler::runImpl()::$_6::operator()() const obj-x86_64-linux-gnu/../src/Server/TCPHandler.cpp:318:25 (clickhouse-tsan+0x1612cf54)
Previous write of size 8 at 0x7b24000ad818 by thread T229:
0 DB::WriteBuffer::next() obj-x86_64-linux-gnu/../src/IO/WriteBuffer.h:43:15 (clickhouse-tsan+0x1612b793)
1 DB::TCPHandler::sendLogData(DB::Block const&) obj-x86_64-linux-gnu/../src/Server/TCPHandler.cpp:1561:10 (clickhouse-tsan+0x1612b793)
2 DB::TCPHandler::sendLogs() obj-x86_64-linux-gnu/../src/Server/TCPHandler.cpp:1632:9 (clickhouse-tsan+0x161228ab)
3 DB::TCPHandler::runImpl()::$_1::operator()() const obj-x86_64-linux-gnu/../src/Server/TCPHandler.cpp:229:62 (clickhouse-tsan+0x1612c4e1)
```
</details>
Before this patch INSERT via Buffer/Kafka may re-use previously set user
for that connection, while this is not correct, it should reset the
user, and use global context.
Note, before [1] there was a fallback to default user, but that code had
been removed, and now it got back.
[1]: 0159c74f21 ("Secure inter-cluster query execution (with initial_user as current query user) [v3]")
Also note, that context for Buffer table (and others) cannot be changed,
since they don't have any user only profile.
I've tested this patch manually using the following:
create table dist (key Int) engine=Distributed(test_cluster_two_shards_secure, default, data, key);
create table buffer (key Int) engine=Buffer(default, dist, 1, 0, 0, 0, 0, 0, 0);
create table data (key Int) engine=Memory();
# to start the connection with readonly user
$ clickhouse-client --user readonly -q 'select * from dist'
$ clickhouse-client -q 'insert into buffer values (1)'
# before this patch this produces errors like:
# 2021.09.27 23:46:48.384920 [ 19474 ] {} <Error> default.dist.DirectoryMonitor: Code: 164. DB::Exception: Received from 127.0.0.2:9000. DB::Exception: readonly: Cannot execute query in readonly mode. Stack trace:
v2: reset the authentication instead of using default user (as suggested by @vitlibar)
v3: reset Session::user and introduce ClientInfo::resetAuthentication (as suggested by @vitlibar)
v4: reset the session every time in interserver mode (suggested by @vitlibar)
