Add --evict-older-than (#605)

Add --evict-older-than (#605)

The argument adds another mechanism to control contents of cache directory. And
is based on the LRU tracking behaviour.

As of now there is no way for ccache to eliminate files which were
are no longer needed. As a result these files stay and are kept around until
either max_files/max_size is reached.

If a particular project is being built regularly but for some reason is allowed
to grow in size, then under such circumstances using the LRU mechanism to control
cache size, lends as perfect solution.

The argument takes a parameter N and performs a cleanup.
While performing cleanup the oldest file in ccache can
only be N seconds old. However this cleanup will not take max_files and
max_old into consideration