Fix bootstrap chicken and egg condition #50

[brendanheywood]

Closes #50

[matthewhilton]

I tested this on my local instead just watching redis and hitting the front page after purging caches, and it appears to have stopped a whole load of other keys appearing in redis:

Before	After

Some of these keys though look like lang string storage that is no longer being stored, which is interesting.
Checking the debug info, it looks like this is causing any cache that is not the config cache to use no caching, even on subsequent loads.

Before	After

Quickly stepping through with xdebug on I can see its hitting https://github.com/catalyst/moodle-tool_forcedcache/pull/51/files#diff-d265ec6c9603db572b0789e13a6e79a2b16bbf7e7ceef4b489ff497c7a9bb7dcR65 a lot while trying to make cache stores, which is strange since it should be set on the second request since the site identifier bootstrap file exists.

[matthewhilton]

Turns out I had $CFG->siteidentifier = '' in my local which was causing it to never use the cache now with this patch which makes sense, it was doing its job as intended. After removing this it uses the cache after the first load as expected.