Add inode cache for file hashes (#577)

Add inode cache for file hashes (#577)

The inode cache is a process shared cache that maps from device, inode,
size, mtime and ctime to saved hash results. The cache is stored
persistently in a single file that is mapped into shared memory by
running processes, allowing computed hash values to be reused both
within and between builds.

The chosen technical solution works for Linux and might work on other POSIX
platforms, but is not meant to be supported on non-POSIX platforms
such as Windows.

Use 'ccache -o inode_cache=true/false' to activate/deactivate the cache.