trigger keystroke timing obfucation only if the channels layer enqueud

trigger keystroke timing obfucation only if the channels layer enqueud
some data in the last poll() cycle; this avoids triggering the
obfuscatior for non-channels data like ClientAlive probes and also
fixes a related problem were the obfucations would be triggered on
fully quiescent connections.

Based on / tested by naddy@