getCustomFieldID switch to API, add caching, add full string return option
In most of my extensions I implement something like this: https://github.com/mattwire/uk.co.mjwconsult.recurmaster/blob/master/CRM/Recurmaster/Utils.php#L14
Because I want the lookup to be cached so I can use it multiple times without a performance issue (eg. within hooks). It is also useful to be able to return the full string equivalent (ie. custom_123 instead of 123) as it can then be used directly in parameter arrays like $key = getCustomFieldID()
without doing: $key = 'custom_' . getCustomFieldID()
This is a proposal to update the core function getCustomFieldID so that it is more useful. It should also help towards #109 (closed)