Coordinate changes to channel-logs

All appends to a channel-log funnel into a channelLogWriter object that
collects them and writes them at once. This will improve performance of
heavily-loaded single-node configurations.
Fixes #168
channelLogWriter now sorts changes by seq before appending them

Also introduced a 50ms pause between updates of a channel-log, so
more changes get batched together.
Updated go-couchbase. Fixed unit tests.