Add connection check to Trilogy

[jhawthorn]

Same as #134 but with requested Ruby API change: always raises an exception on error, and name is #check.

This adds a very cheap connection check to Trilogy. This cheap check can be used before a connection for example is checked out from a connection pool.

It allows for making the client more robust in the case of the server having disconnected (like a failover, an intermediate proxy like ProxySQL or VTGate restarted etc) but it can be reconnected safely.

It works by doing a non-blocking recv to peek if there's any data. If there is, or if the function would otherwise block it means the connection is still safe.

If the TCP connection has been closed, recv immediately returns with zero bytes read. This indicates the connection is no longer safe to use. We do this directly on the file descriptor to be consistent regardless of things like TLS being used.