omcache: more improvements towards the first release

* all command apis now accept a timeout_msec argument which specifies how
  long to block waiting for a response to the query; -1 means indefinitely
  and 0 means no waiting at all.  if we're not waiting at all the commands
  are sent in 'quiet' mode when applicable to allow server to batch
  responses when we finally want to read them
* added CAS operation to omcache_set
* omcache_set_servers now replaces the server list instead of appending it
* renamed retry timeout to reconnect timeout and io timeout to dead timeout
  and made both configurable over the api
* added omcache_server_info api
license: switch to apache 2.0