Add a setting to enable Purge success logging

Problem/Motivation

Every time a purge is successful, either purgeKey() or purgeAll(), a new info log message stating it is added to the database.

While in development or QA, that is useful for us to make sure it is working. Once in production, the number of log entries are expected to grow exponentially, flooding the logs and most likely causing the watchdog tables to consume a lot of database storage.

Proposed resolution

Given the debugging aspect of the messages, a checkbox should be added to the modules configuration page where this logging functionality can be optionally enabled.

Logging the critical should remain mandatory, as it's an indicator whether fastly can be reached or not.

User interface changes

A new checkbox got added to Purge options fieldset in the configuration page.

API changes

N/A

Data model changes

A new configurtipon variable purge_logging was added to the schema.

Release notes snippet

Logging messages for successful Purges are now disabled by default. It should be manually enabled in the module's configuration page when nedded.