Prevent broken concurrent schema pulls

There's a race condition around pulling schema changes, that can occur in case the schema changes push/propagation mechanism is not immediately effective (e.g. because of network delay, or because of the pulling node being down, etc.).

If schema changes happen on node 1, these changes do not reach node 2 immediately through the SCHEMA.PUSH mechanism, and are first recognized during gossiping, the corresponding SCHEMA.PULL request from node 2 can catch the node 1 schema in the middle of it being modified by another schema change request. This can easily lead to problems (e.g. if a new table is being added, and the node 2 request reads the changes that need to be applied to system_schema.tables, but not the ones that need to be applied to system_schema.columns).

This PR addresses that by synchronizing the SCHEMA.PULL "RPC call" executed in node 1 by a request from node 2 with the method for applying schema changes in node 1.