module: cache stat() results more aggressively

Reduce the number of stat() system calls that require() makes by caching
the results more aggressively.

To avoid unbounded growth without implementing a LRU cache, scope the
cache to the lifetime of the first call to require().  Recursive calls
(i.e. require() calls in the included code) transparently profit from
the cache.

The benchmarked application is the loopback-sample-app[0] and it sees
the number of stat calls at start-up go down by 40%, from 4736 to 2810.

[0] https://github.com/strongloop/loopback-sample-app

PR-URL: https://github.com/nodejs/node/pull/4575


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>