Fix the unintentional CredProvider replace.

As Benjamin spotted, the HttpClientBuilder#setDefaultCredentialsProvider
method silently replaced any previously set provider.

This also happened in case when remote but also proxy needed
credentials. In worst case, the provider would be replaced 3
times (remote authc + HTTP + HTTPS proxy).

Added Javadoc warning about this, and also added
a new Builder method that delegates to it's own instance
of CredentialsProvider that if used, will replace the
HttpClientBuilder.

Added UT verifying the change.
Shiro since release 1.2.0 supports
authc and authz caching, that we use
instead of the removed (removed when
upped Shiro version to 1.2.0) homegrown
caching.
Merge pull request #373 from sonatype/realm-caches

NEXUS-6287: Cache use cleanup
NEXUS-6197: Update delivered LVO config
Merge pull request #400 from sonatype/fix-credentials-override

Fix the unintentional CredProvider replace.
Merge remote-tracking branch 'origin/master' into quartz-scheduler