connpool: remove broken connections and ensure that the existing connection is for the intended instance

[ImeevMA]

After #9892 is pushed to master we will have new problems:

1. it's possible that the number of connections, including broken ones, will become more than 2^26 or Lua will run out of memory, so we need a mechanism to clean up broken or unnecessary connections.
2. it is possible that the URI of the instance will change, but the connection will not be broken. In this case, we need to determine that the connection is not actually for the instance and create a new one.

[R-omk]

#9563 related