Use APCu by default, if available

Problem/Motivation

Crawler rate limit module currently requires backend to be configured before rate-limiting can be used. Until user enters module configuration into the settings.php file, rate-limiting is disabled. It would be nice if we could detect if APCu is enabled and configure it for bot rate-limiting as soon as the module is installed.

Proposed resolution

When module is installed, check if APCu extension is enabled. If APCu is detected and no settings are provided in the settings.php, enable bot rate-limiting with sensible default values for number of requests allowed in the given time interval. Once the user adds module configuration to the settings.php those settings take precedence and override defaults.

Remaining tasks

Work out the details and provide patch.

User interface changes

None.

API changes

None.

Data model changes

None.