Do not schedule connection retry on dead eventloopgroup

[m0wfo]

ClientBootstrap.connectWithRetry(...) should check to see if the bootstrap's event loop group is still available for scheduling as it may have been shut down (e.g. by an int signal) between attempts. If the event loop is dead, no further retries should be attempted. Update documentation to make this clear.

This is to avoid the error:

ERROR: Cannot schedule tasks on an EventLoop that has already shut down. This will be upgraded to a forced crash in future SwiftNIO versions.

(plus backtrace when not compiled for release)