New SSE implementation

Fixes #205 #209 #220 #222

#205 (Move SSE related classes to http): This is a new implementation of SSE for some optimizations and spec compliance.
#209 (Deprecated all SSE classes in text pkg): SSE is only applicable to HTTP.
#220 (SSE Output confusion): Default case only emits data event.
#222 (Improved SSE API): Better construction semantics.
#30 (Optimized SSE decoder): Rewrite of the existing decoder.
#267 (Change ServerSentEvent to store data as `ByteBuf`): This is in the new implementation so it is not backward incompatible.