change supportsPrewhere() of table engine Buffer and return its destination_table supportsPrewhere() or not

Function trigramDistance() added for string similarity search

Fix flap on kill query test

Fixed ThreadPool when there are too many inactive threads #4485

Speedup ddl cleanup

Support xxHash with external lz4

Add performance test for benchmark from the website

External dict integration test

xxhash.h does not exist in external lz4 because it is an implementation detail
and its symbols are namespaced with XXH_NAMESPACE macro.  When lz4 is external,
xxHash has to be external too, and the dependents have to link to it.

This find_xxhash.cmake is based on find_lz4.cmake.

Do not create part_log if it is not defined in configuration file

Refactoring: simplify InDepthNodeVisitor

Added 'query_thread_log' section to configuration file. Now query_thread_log doesn't work without this section.

mysqlxx::Pool is full checking exception fixed