This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Use a dedicated thread to run Scheduler::Defer
This avoids blocking of a worker during operations that require waiting.
In particular uploads risked blocking a unicorn.
This also add a queue "length" that discourse prometheus consumes.