Fix a bug in cleanupUnneededSnapshots()

Summary: Before this diff cleanupUnneededSnapshots() didn't actually remove cleaned-up snapshots from _snapshotMap. That means we leaked all snapshots and only removed them at the end, in function dropAllSnapshots(). This bug caused couple of test failures in the CI tool.

Test Plan:
I used the test from the Mongo CI tool that was failing:

    python buildscripts/resmoke.py --suites=core_small_oplog_rs --storageEngine=rocksdb --jobs=1 --continueOnFailure --log=buildlogger --reportFile=report.json

The failure reproed locally consistently. After this diff, the test is successful.

Reviewers: IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D48393

src/rocks_snapshot_manager.cpp

@@ -68,7 +68,12 @@ namespace mongo {
         }
 
         auto it = _snapshots.begin();
-        for (; it != _snapshots.end() && *it != _committedSnapshot; it++);
+        for (; it != _snapshots.end() && *it != _committedSnapshot; it++) {
+            auto snapshotHolder = _snapshotMap.find(*it);
+            // _snapshots and _snapshotMap have to have the same snapshots
+            invariant(snapshotHolder != _snapshotMap.end());
+            _snapshotMap.erase(snapshotHolder);
+        }
         invariant(it != _snapshots.end());
         _snapshots.erase(_snapshots.begin(), it);
     }