ChannelFlushPromiseNotifier should allow long value for pendingDataSize

Motivation:
At the moment ChannelFlushPromiseNotifier.add(....) takes an int value for pendingDataSize, which may be too small as a user may need to use a long. This can for example be useful when a user writes a FileRegion etc. Beside this the notify* method names are kind of missleading as these should not contain *Future* because it is about ChannelPromises.

Modification:
Alter add(...) method to take a long for pendingDataSize. Rename all *Future* methods to have *Promise* in the method name to better reflect usage.

Result:
ChannelFlushPromiseNotifier can be used with bigger data.