Fixed Wrong Code in Mutations That May Lead to Memory Corruption

Fixed wrong code in mutations that may lead to memory corruption. Fixed segfault with read of address 0x14c0 that may happed due to concurrent DROP TABLE and SELECT from system.parts or system.parts_columns. Fixed race condition in preparation of mutation queries. Fixed deadlock caused by OPTIMIZE of Replicated tables and concurrent modification operations like ALTERs. #6514 (alexey-milovidov)