- Mark getCreateTableQuery() non virtual to call it from ctor
- Make check for CREATE TABLE simpler (by caching result)
- Extend error message.
v2: at SystemLog tables has not been initialized yet, so SHOW CREATE
cannot be obtained for the existing table, move the check to the
prepareTable() call.
v3: reset UUID and SETTINGS of MergeTree in CREATE TABLE query
v4: fix re-creating table with the same UUID
