Rowcache invalidator launch revamp

- Rowcache invalidator is now launched as part of query service
- vtocc can also launch rowcache invalidator, with new flag binlog-path
- Explicit flags: enable-rowcache, enable-invalidator
- vtocc also needs dba credentials for invalidator
- New sync2.ServiceManager for managing goroutine start & stop
Merge branch 'rowcache'
Small fix in error message.
Merge branch 'master' of github.com:youtube/vitess