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

[sharonovd]

Corner cases:

1. What should be done if after a list refresh currently active node is no longer in it?
2. What is the exact API for connector and tarantool?

@Totktonada please help elaborate those, I usually tend to simplest solutions