ZF-4670: Add file path cache to Zend_Loader::loadFile()

Description

The biggest bottleneck of plugin loading is due to cases when the file is not found on the given paths; when it is not, an error is pushed onto the PHP error stack -- a very expensive operation that often takes up to 10 times as long as the actual include() call.

A file path cache should be added to Zend_Loader::loadFile() to allow caching the success or failure of loading so that in subsequent requests for the given path/file combination, the method may return early if the combination will not resolve.

Comments

Added to trunk with r12067

Original implementation led to performance degradation under normal use cases.

includeFileCache added to PluginLoader, netting around 64% performance gains in real-world applications.

Changing issues in preparation for the 1.7.0 release.