Remove multi-shard locks

We currently take `ShareUpdateExclusiveLock`s on all shards involved in a multi-shard command (DDL, `INSERT ... SELECT`, `master_modify_multiple_shards`, multi-row INSERT), which prevents concurrent multi-shard commands on the same shards in order to avoid distributed deadlocks. We could now instead rely on distributed deadlock detection and allow a greater degree of concurrency.

Removing the multi-shard locks also allows us to run multi-shard commands (except DDL) from workers in MX.