secache: Store and retrieve cached values from single file

This class can be used to store and retrieve cached values from single file.

It can store one or more keys in a single cache file.

The class can also look up for a given key and retrieve the store value.

It can lock the file during access to prevent corruption, but there is also a sub-class that can be used to access the cache file without locking it.

It can automatically clean-up the least recently used entries to free space.