Introduce system for garbage collecting connection managers

[brandur]

Introduces a system for garbage collecting connection managers in an
attempt to solve #850.

Previously, the number of connection managers (and by extension the
number of connections that they were holding) would stay stable if a
program used a stable number of threads. However, if threads were used
disposably, the number of active connection managers out there could
continue to grow unchecked, and each of those could be holding one or
more dead connections which are no longer open, but still holding a file
descriptor waiting to be unlinked in disposed of by Ruby's GC.

This PR introduces a connection manager garbage collector that runs
periodically whenever a new connection manager is created. Connection
managers get a timestamp to indicate when they were last used, and the
GC runs through each one and prunes any that haven't seen use within a
certain threshold (currently, 300 seconds). This should have the effect
of removing connection managers as they're not needed anymore, and thus
resolving the socket leakage seen in #850.

I had to make a couple implementation tweaks to get this working
correctly. Namely:

• The StripeClient class now tracks thread contexts instead of
  connection managers. This is so that when we're disposing of a
  connection manager, we can set default_connection_manager on its
  parent thread context to nil so that it's not still tracking a
  connection manager that we're trying to get rid of.

• StripeClient instances can still be instantiated as before, but no
  longer internalize a reference to their own connection manager,
  instead falling back to the one in the current thread context. The
  rationale is that when trying to dispose of a connection manager, we'd
  also have to dispose of its reference in any outstanding
  StripeClient instances that might still be tracking it, and that
  starts to get a little unwieldy. I've left #connection_manager in
  place for backwards compatibility, but marked it as deprecated.

r? @ob-stripe
cc @stripe/api-libraries

[ob-stripe]

LGTM. As usual, thanks for leaving good comments 👍

[brandur-stripe]

Thank you OB!

I made one more minor tweak in reducing the expiry age of a connection manager from 300 to 120 seconds. I think it makes sense to try and prune them a little more aggressively in case a lot of threads are being created and churned through, and it's not a big deal if a thread that's not used very often has to occasionally recreate its connection manager.

[brandur-stripe]

Released as 5.2.0.

[jodosha]

Hello folks, before to upgrade to 5.2.0, I had a look at the changes introduced with this new version.
There is something that caught my attention: the way you try to calculate time diff.

Time.now (wall time) isn't stable, and it can be unpredictably change (even backwards in time) during an operation is performed. You should use a monotonic clock instead.

Here's a detailed explanation of the problem: https://blog.dnsimple.com/2018/03/elapsed-time-with-ruby-the-right-way/

I hope that my feedback was helpful. Thanks for Stripe and this gem. 💚

[ob-stripe]

Hey @jodosha, thanks! This is definitely useful feedback. I'll let @brandur-stripe have the final say, but it sounds to me like we should be using a monotonic clock as you pointed out.

[brandur-stripe]

Thanks @jodosha! I took a crack at moving us to monotonic time in #857.

[jodosha]

Thank you all for considering my feedback.