feat: manual replication leader switchover

Summary

• Change system table _pico_replicaset format:

  • remove master_id field
  • add current_master_id field
  • add target_master_id field

• Add support for manually switching a given replicaset's master. Currently this can be done via cas dml request by changing the target_master_id of a given replicaset.

• Add a new replication leader switchover step to the governor which is run if current_master_id != target_master_id:

  • if current master is not offline then make it read_only = true and get it's current vclock value
  • if current master is not offline then make the target master wait until it's vclock is synced with the current master's one
  • make the target master read_only = false
  • update _pico_replicaset such that target_master_id = current_master_id

Ensure that

Closes #297 (closed)

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• (if necessary) A follow-up doc issue is created in picodata/docs and linked here