Implement caching option in RDS protocol

[manugarg]

Implement caching option in RDS protocol.

RDS client should be able to ask RDS server to return resources only if resources have changed since the provided timestamp.

..Request {
  ...
  optional int64 if-modified-since
}

If RDS provider supports if-modified-since, it will add resources to response only if they have changed since the provided timestamp. Response will also include the last-modified timestamp so that client can verify that server actually supports the if-modified-since option.

..Response {
  ...
  optional int64 last-modified
}

Ref: #634

[manugarg]

This should be done now:

1. RDS protocol has support for caching now.
2. RDS file provider supports caching, based on the file's modified_time timestamp.