API versioning

[sachinholla]

1) Enhanced REST server to accept the client version through new request
header 'Accept-Version'. This is an optional header. If specified, it
should be of Translib's yang bundle version format "Major.Minor.Patch".
Server returns 400 status if Accept-Version value is invalid.

2) Accept-Version header value is passed as "ClientVersion" value to the
translib APIs. Translib version check errors are mapped to 400 status
with error-type "protocol", error-tag "invalid-value" and an error-info
json object indicating allowed version range (server version and server
base version).

Sameple error response payload:

  {
  "ietf-restconf:errors" : {
    "error" : [
      {
        "error-type" : "protocol",
        "error-tag" : "operation-not-supported",
        "error-message" : "Unsupported client version 0.0.1",
        "error-info" : {
          "version-error" : {
            "ServerBaseVersion" : "1.0.0",
            "ServerVersion" : "2.1.3",
            "ClientVersion" : "0.0.1"
          }
        }

3) Translib version checks are not performed if client did not send the
Accept-Version header.