Changing get version
- get version will be a HTTP GET request with a special header
  "X-VOLD-Get-Version" with non null value
- get version response body will containg the vector clock(s)
- deleting RestServerGtVersionRequestValidator.java
- RestServerGetRequestValidator will handle get, getall and getversion
  operations

added non negative checks to timestamp and timeout headers
- Added config parameter for enabling Rest Service. Rest Service will run
on port 8085 but will not be enabled by default. Set rest.enable=true to
bring up RestService.
- Bringing back HttpService on it port 8081.
- Changing the Rest Service unit tests to use port 8085