Use the new improved JSServer from Walrus

This allows sync() functions to run on multiple goroutines at once,
improving parallelism.
If no sync fn configured, don't install a default one

Having a nil ChannelMapper has the same effect as having one with the
default JS function, but is a lot faster to run (no Otto overhead.)
Optimized channel-log updates

Switched to a lower-level data format that is fast to encode/decode, can
be scanned and modified without fully unpacking it into Go values, and
can be added to by simply appending bytes.

This is a huge performance improvement over JSON channel-logs: something
like 5-10x faster at importing a sample CouchDB database.
Set GOMAXPROCS to use all CPUs if not configured in the environment

For some reason Go will use only 1 CPU by default unless the env var
GOMAXPROCS is set. At launch time, raise this default to the number of
available CPUs.