Play buffer when reattaching to channel

[emagaliff]

When a channel is detached, the buffer is not tracked or saved by playback, and thus not played when reattaching. It would be great if the module could track all channels and play back the entire buffer for a detached channel (or from the last requested buffer, if possible).

[Mikaela]

Fix would also close jpnurmi/znc-chanfilter#4 which I originally reported to wrong place and never got closed.

[ghost]

@jpnurmi Bump.