Summary:
`MountingCoordinator::waitForTransaction()` allows waiting for a new coming transaction synchronously, as efficient as waiting for a mutex.
This feature can be used to implement more high-level synchronous rendering APIs.
Reviewed By: mdvacca
Differential Revision: D17629425
fbshipit-source-id: acd91b941e4d6d43bc4518f332a1604e14506be9