Fix memory leaks in watch and callback API-s

[keijokapp]

Resolves #82

Apparently, the references created in watchFuture and fdbFutureToCallback have to be deleted explicitly in addition to being unref-ed. Just deleting wouldn't be enough either. After applying this patch, the memory usage seems to eventually flatline and Valgrind is reasonably happy.

From Node-API doc:

References must be deleted once they are no longer required by the addon. When a reference is deleted, it will no longer prevent the corresponding object from being collected. Failure to delete a persistent reference results in a 'memory leak' with both the native memory for the persistent reference and the corresponding object on the heap being retained forever.

[josephg]

Good catch! Thanks for figuring this out.