Previously, the Review Board server was not generating correct caching

information, resulting in the client side cache not being updated correctly.

Now that it has been fixed server side, the RBTools client needs to perform a

version check to see if the cache should be disabled.
The HTTP cache is now disabled by default and will be enabled upon the first

request to the root resources (/api/), which contains the version number of

the server, if the server is at least the minimum supported version. Currently

the minimum supported version is 2.0.13.
The Transport class and its subclasses now have the machinery available to

enable the API cache.
The HTTP cache for the API now appends both If-None-Match and

If-Modified-Since headers if caching information for both is stored.

Prevously, only one of those headers would be used.

Ran unit tests.
Ran rbt status -d against https://reviews.reviewboard.org and saw that the

request to the /api/ endpoint did not cause a caching logging message, but

all subsequent requests did. This is the expected behaviour as the server is

currently in version 2.1 alpha 0 (dev).