[+FEATURE] FLOW3 (Cache): Implement igbinary support
This changeset adds support for the serialization accelerator
igbinary. If the igbinary extension is loaded, the VariableFrontend
of the cache framework will use its serialize() and unserialize()
functions instead of the PHP built-in variants.
Depending on the amount and nature of data, this speeds up cache
serialization by about 100%.