Research: PostgreSQL protocol support

The research was initiated by @kostja.

We need to find out a possibility of the libpq protocol support in sbroad (picodata implementation). Ideally each client should keep connections to all nodes in the cluster and balance the queries via instances. Also in this ideal world of ponies and rainbow the client caches the bucket map and can calculate buckets by itself.

1. https://github.com/sfackler/rust-postgres
2. https://www.postgresql.org/docs/current/protocol-flow.html
3. https://www.postgresql.org/docs/current/libpq.html
4. https://www.postgresql.org/docs/current/libpq-async.html
5. https://www.tarantool.io/en/dev/core/cbus/
6. https://github.com/tarantool/tarantool/blob/master/src/box/iproto.cc
7. https://www.tarantool.io/en/doc/latest/dev_guide/internals/iproto/requests/#box-protocol-auth
8. https://www.tarantool.io/en/doc/latest/dev_guide/internals/iproto/sql/#box-protocol-execute