Get a list of 'connectable' instances from the first node

[sharonovd]

Use-case:
there is a sharded cluster of ~50 tarantools, 12 vshard-routers. User wants to be able to reconnect to another noe smoothly (if the first one goes down). It is not suitable to pass all 50 nodes to connectmesh, moreover - adding a new router requires all client applications restart.

Proposed solution:

connector should receive a list of connectable nodes from the first one it connects to. Exact procedure on tarantool side must be configurable.
this list should replace the one passed initially.
this list should be refreshed periodically in the background