Don't return doc_count in GET /db: it's too expensive to compute

Fixes #278
Logging and expvar improvements

* 'Bucket' log key now shows return value for Update operations
* 'HTTP+' logging now shows elapsed time more accurately
* New expvar 'syncGateway_httpListener' for the base/http_listener code.
  It logs the max number of simultaneous HTTP sockets and the max wait
  for a socket to be accepted.
* New expvars 'syncGateway_rest/requests_????ms' that provide a histogram
  of how long HTTP handlers took to run.