gitweb/lib - Use CHI compatibile (compute method) caching interface

gitweb/lib - Use CHI compatibile (compute method) caching interface

If $cache provides CHI compatible ->compute($key, $code) method, use it
instead of Cache::Cache compatible ->get($key) and ->set($key, $data).
In the future other compatibile but differently named methods, like
Cache::FastMmap's ->get_and_set($key, $code) method, could also be
supported; though CHI which has ->compute() includes CHI::Driver::FastMmap
wrapper.

GitwebCache::SimpleFileCache provides 'compute' method, which currently
simply use 'get' and 'set' methods in proscribed manner.  Nevertheless
'compute' method can be more flexible in choosing when to refresh cache,
and which process is to refresh/(re)generate cache entry.  This method
would use (advisory) locking to prevent 'cache miss stampede' (aka
'stampeding herd') problem in the next commit.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>