Twisted already closes file handles for us, doing it again makes OSX sad sometimes.
Cap the rlimits we set to the existing limits to avoid accidentally trying to raise them.
Split Rlimiter into a standalone module to reduce the amount of stuff it pulls in and bring back the fd closing code.
Merge branch 'feature/issue-524-respect-maximum-rlimits' into develop
Merge branch 'develop' into feature/issue-519-sandbox-api-should-catch-uncaught-exceptions-when-dispatching-requests