Citus continues sending commands to workers which have failed

All the connection functions which send commands do not check whether the connection is in a failed state before sending the command. This means that if we send a `BEGIN` in `StartRemoteTransactionBegin` which fails, for instance, later functions will continue to try sending commands to the remote node. If the failure was just temporary this could lead to big problems!

Example here: https://github.com/citusdata/citus/pull/1169/files#diff-59c96442a9866224ac2cfe12cc457f37R2071

Discussion here: https://github.com/citusdata/citus/pull/1169/files#r97957740