Avoid segfaults and many other problems related with modifying global context in DDLWorker. [#CLICKHOUSE-3260]

Don't leave trash part nodes in ZK in case of deleting non-existing duplicated parts. [#CLICKHOUSE-3252]

This reverts commit f306ccde.

It is used by debian/copy_clang_binaries.sh

std::random_shuffle() may or may not use std::rand() which isn't
seeded, and since configuration parsing runs predictably as one
of the first things after server startup, the list of ZK nodes
is shuffled the same way on all replicas.