feat: prevent concurrent schema changes on master by using a global lock

A global lock for schema changes that solves the following issue:
An expensive index creation can cause RPC timeouts, resulting in re-sending of the
same RPC even though the operation is still in progress on the master. The second RPC
attempts to create the same index, but it realizes that the index already exists
even if the operation has not been completed yet. To handle this scenario, a global
lock was added in `proc_apply_schema_change` to prevent concurrent schema changes.