close persistent connections before forking

An attempt to fork without causing broken SSL connections. In testing with @hone, using threads instead of forking has a roughly 50% performance penalty. Looks like Gem::Installer spends a lot of time doing CPU-bound ungzipping and the like. So we're trying to make forking work even with SSL connections.

Please to tell me if this is crazy, @drbrain, @hone, @evanphx, @skottler.