Linda fixes

* bumped version to 3.7.7
* fix crash when calling linda:count() on unknown keys
* purge key storage with linda:set( key, nil) on an unlimited key to
reduce memory usage with lots of keys
* linda:limit() wakes write-blocked threads if necessary when the new
limit enables writes to occur again
* linda:set() wakes write-blocked threads if necessary if the operation
created some room to write into