Prepare for LLVM-11

Replace useless multiset with unordered_set

Tests/partition pruning

Query obfuscator

Fix ASTColumnsReplaceTransformer::Replacement::clone()

Fix flaky integration test

Fix race condition between write ahead log cleanup and rename

Fix for default database race on server startup

Update ForkProcessor.cpp

Added Comprehension to Improve Performance

fix build bug for merge_selector2.cpp

Fail more quickly in clickhouse-test if server does not respond

Put createBlockForSet declaration to header file.