Use "/path" RPC routes to call plugin-defined endpoints.
Use ".proc" RPC routes to call builtin picodata endpoints (for example
.proc_instance_info)

- use RouteBuilder to register rpc endpoints (input and ouptut is raw
  bytes for now)
- use RequestBuilder to send RPC requests to other instances which are
  running a specified plugin.service (guarded by version)
    - supported routing by
        - explicit instance_id
        - replicaset_id
        - bucket_id
        - any available instance with given service
- use context::Context to extract information about request or add
  custom fields at runtime (middleware not yet implemented)

The problem is MsgTimeoutNow always results in a new election, because
it explicitly ignores the pre_vote step. This results in integration
test flakiness whenever a follower calls promote_or_fail while the
leader's raft log is longer.

- update sbroad submodule to commit with helper
patches for dml on global tables
- add support for sql dml insert/update/delete
on global tables. Insert on conflict is not
supported yet
- Current implementation uses a single batch of
dml commands via CAS. No additional predicates
are used in CAS, so no isolation level is guaranteed
even when query only refers to global tables

1. make it non-nullable
2. fix tests