RPCs invoked with SYNCRONOUS_IGNORE_LEAVERS taking too long if one of the targets had already left

We were waiting for responses from the leavers as well, and then we were ignoring the timeout. Instead we should remove the leavers from the targets list before sending the command.

`SYNCHRONOUS_IGNORE_LEAVERS` is used for `CommitCommand`s and `CacheViewControlCommand

`s, if this happens it will block the cache view installation (and so the entire cluster) until the RPC timeout expires.