Decorate OrderedExecutor threads rather than runnables

[ivankelly]

Actually, decorate the runnables too, but apply the decoration in a
decorator on the thread so that if a runnable/callable is passed to
the thread, the decoration will be applied.

This allows users of OrderedExecutor/Scheduler to use
executor.chooseThread() for CompletableFuture async handlers and still
get things like the MDC context.

[ivankelly]

This is blocking tests passing in #1646 .

[dlg99]

LGTM.
There is one related item to the executor story (not for this PR, of course): for each runnable we create new objects (depending on configuration, timed and/or context preserving runnables). Would be nice to use recycler for them.

[sijie]

for each runnable we create new objects (depending on configuration, timed and/or context preserving runnables). Would be nice to use recycler for them.

+1 for using recycler to improve them.

[ivankelly]

👍 Recycler sounds good. In general, the executor stuff could do with a bit of a refactor. The way OrderedScheduler inherits from OrderedExecutor rather than a common base is strange and means OrderedScheduler needs to do a bunch of janky casts. Once I clear my outstanding patch series I might take a look at it, along with other thread model stuff.