Connection pool mechanism

[pavlua]

• added a few non-breaking changes in Box/IBox public interface (Do-methods and ConnectionGoesDown event handler), which are required for pooling implementation
• ConnectionPool class implemented, user can work with multiple instances of Tarantool in unified way (and use RequestMode to control request routing to ro/rw instances)
• ConnectionPool use round robin logic
• ConnectionPool use abstraction ITarantoolNodesSource in initial filling, so user can easily change the source of Tarantool nodes: config, etcd, etc
• ConnectionPool has autoreconnection logic: if node went down, it eliminates from pool, and special background reconnector starts to monitor its state and try to return it to pool back
• unit tests
• minor file reorganization: classes that implements inner Box machinery moved into Core folder

what is not implemented yet

• readme updates with example(s)
• monitoring of ro/rw state of nodes (ro can become rw after autofailover for example, ConnectionPool should update its state)
• autorenewal of nodes in pool, by timer for example (we can use ITarantoolNodesSource abstraction)
• zones/distances feature: ability to assign a zone/distance to each node and instruct pool to use the closest nodes as possible

[oleg-barash]

Flaky test. For some reason sometimes this test fails with null-reference here