journal: separate out read-only peekLatest

Summary:
I have realized that Sapling's (and other things') use of getJournalPosition() has an unwanted side effect of setting lastModificationHasBeenObserved=true, which has interplay with EdenFS subscribers. Sapling isn't really observing anything, so we don't want to cause any disturbances.

Fix by separating getLatest to observeLatest (which sets lastModificationHasBeenObserved=true) and peekLatest. Since peekLatest doesn't write, we can also separate into a read/write lock to avoid the exclusive mutex acquisition in getJournalPosition.

The only caller I change in this commit to peekLatest is eden's use building its "status" cache key. I wanted to also change getCurrentJournalPosition (which is what "sl debugwait" calls), but it is necessary/intentional for getCurrentJournalPosition to set lastModificationHasBeenObserved=true based on callers' expected semantics.

Reviewed By: MichaelCuevas

Differential Revision: D95146037

fbshipit-source-id: ff658b57f1ebcb5e61d8fea4915cbd243bd8dbbb

Author: muirdm

eden/fs/service/streamingeden.thrift

@@ -156,7 +156,6 @@ service StreamingEdenService extends eden.EdenService {
    * never calls changesSinceV2 or getCurrentJournalPosition in
    * response to a notification on this stream, future notifications may not
    * arrive.
-
    */
   stream<eden.JournalPosition> streamJournalChanged(
     1: eden.PathString mountPoint,