thrift: Use query_processor::get_migration_manager()
thrift: Use query_processor::get_migration_manager()

Thrift needs migration manager to call announce_<something> on
it and currently it grabs blobak migration manager instance.

Since thrift handler has query processor rerefence onboard and
the query processor can provide the migration manager reference,
it's time to remove few more globals from thrift code.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla