Specifically, clean up if the thread is killed while it's blocked
awaiting the lock... if we get killed on some other arbitrary line, the
result remains quite undefined.
Specifically, the "loose upgrades" behaviour that allows us to obtain an
exclusive right to load things while other requests are in progress (but
waiting on the exclusive lock for themselves) prevents us from treating
load & unload interchangeably: new things appearing is fine, but they do
*not* expect previously-present constants to vanish.
We can still use loose upgrades for unloading -- once someone has
decided to unload, they don't really care if someone else gets there
first -- it just needs to be tracked separately.
The concurrent-ruby gem is a toolset containing many concurrency
utilities. Many of these utilities include runtime-specific
optimizations when possible. Rather than clutter the Rails codebase with
concurrency utilities separate from the core task, such tools can be
superseded by similar tools in the more specialized gem. This commit
replaces `ActiveSupport::Concurrency::Latch` with
`Concurrent::CountDownLatch`, which is functionally equivalent.
