Bug 1018383 - Added request-level caching in NetworkGeolocationProvider.js. r=jdm, a=dougt

This is a cleanup of the patch used in the Tarako bug. I made it clearer that this is request-level caching, NOT position caching.
 This is specifically for preventing unnecessary requests to the server. It is unrelated to the W3C PositionOptions and their effect on position caching.
 Hopefully this detail is clearer in this patch.

Updated mochitests that were relying on old logic to succeed

The failing mochitests were relying on the provider to have a specific behaviour -behaviours that are incidental to the test case. I updated these to reflect the behaviour of the current system.