`ReadYourWritesTransaction` has memory allocated before being passed to
the main thread. This allows both threads to continue to access the
transaction object. Currently, the transaction gets allocated and
initialized on the foreign thread, and then re-initialized on the main
thread. This causes a bunch of extra, unnecessary work for each
`ReadYourWritesTransaction` where the temporary object gets destructed.
The fix is to only allocate memory for the `ReadYourWritesTransaction`
on the foreign thread, and then initialize it once on the main thread.
