gplazma: make plugin creation caching optional

This patch introduces the gplazma.enable.plugin-creation-caching configuration property that
allows the admin to switch off the plugin cache.  This is useful as the cache is arguable a
misfeature and, in its current form, can lead to undesirable behaviour.

A subsequent patch (targeted at master only) will remove this feature.

Target: master
Patch: https://rb.dcache.org/r/7210/
Acked-by: Tigran Mkrtchyan
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: true
Require-book: no