Move 'Request' from SYNC_PROCESSOR to SYNC_CONTEXT

Move 'Request' from SYNC_PROCESSOR to SYNC_CONTEXT

By keeping a local 'Request' value on stack in SyncWorker() to track the last
completed request, we can avoid the need to initiate operations using a per-
processor value and simply use a global one. This means we no longer need
the loops iterating over all SYNC_PROCESSORs in SyncDisableInterrupts(),
SyncEnableInterrupts() and SyncRelease().

Signed-off-by: Paul Durrant <pdurrant@amazon.com>