Automatic master discovery

[Totktonada]

I propose to implement quite simple master discovery algorithm on top of existing connection pool (multi). Key points:

• 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.

Later we'll reimplement it for fresh tarantool versions using watchers instead of polling. Not in the scope of this issue.

Later we'll design a good retry strategy, but it is okay for now to leave it for a user.

mode parameter may be discussed: proposals backed by arguments (not just opinion) are appreciated.

RFC is here.

[Totktonada]

Updated the description with a short proposal what to implement.