Automatic master discovery

[Totktonada]

• Poll box.info.ro on each instance every X seconds.
• Add mode = 'any' / 'ro' / 'prefer_ro' / 'prefer_rw' / 'rw' parameter for call, eval, execute, select, ping, insert, replace, delete, update, upsert.
• Fail fast: if we have no rw instance ATM -> report the fail to a caller.
• Fail fast: got ER_READ_ONLY -> report it to a caller.

RFC is here.

Related: tarantool/go-tarantool#113.