prefer archiving vs deleting sessions.

This gives us a little resiliency to handling messages out of order
across key change.

We still *always* print SN when they change.
And we still verify that the session used for encrypt/decrypt is the trusted
session

// FREEBIE

src/Devices/OWSRecordTranscriptJob.m

@@ -47,7 +47,7 @@ NS_ASSUME_NONNULL_BEGIN
 
     if (transcript.isEndSessionMessage) {
         DDLogInfo(@"%@ EndSession was sent to recipient: %@.", self.tag, transcript.recipientId);
-        [self.storageManager deleteAllSessionsForContact:transcript.recipientId];
+        [self.storageManager archiveAllSessionsForContact:transcript.recipientId];
         [[[TSInfoMessage alloc] initWithTimestamp:transcript.timestamp
                                          inThread:transcript.thread
                                       messageType:TSInfoMessageTypeSessionDidEnd] save];

src/Messages/TSMessagesManager.m

@@ -721,7 +721,7 @@ NS_ASSUME_NONNULL_BEGIN
     }];
 
     dispatch_async([OWSDispatch sessionStoreQueue], ^{
-        [[TSStorageManager sharedManager] deleteAllSessionsForContact:endSessionEnvelope.source];
+        [[TSStorageManager sharedManager] archiveAllSessionsForContact:endSessionEnvelope.source];
     });
 }
 

src/Storage/AxolotlStore/TSStorageManager+SessionStore.h

@@ -7,6 +7,8 @@
 
 @interface TSStorageManager (SessionStore) <SessionStore>
 
+- (void)archiveAllSessionsForContact:(NSString *)contactIdentifier;
+
 #pragma mark - debug
 
 - (void)printAllSessions;

src/Storage/AxolotlStore/TSStorageManager+SessionStore.m

@@ -3,6 +3,7 @@
 //
 
 #import "TSStorageManager+SessionStore.h"
+#import <AxolotlKit/SessionRecord.h>
 
 NSString *const TSStorageManagerSessionStoreCollection = @"TSStorageManagerSessionStoreCollection";
 NSString *const kSessionStoreDBConnectionKey = @"kSessionStoreDBConnectionKey";
@@ -158,6 +159,35 @@ void AssertIsOnSessionStoreQueue()
     }];
 }
 
+- (void)archiveAllSessionsForContact:(NSString *)contactIdentifier
+{
+    AssertIsOnSessionStoreQueue();
+
+    DDLogInfo(@"[TSStorageManager (SessionStore)] archiving all sessions for contact: %@", contactIdentifier);
+
+    __block NSDictionary<NSNumber *, SessionRecord *> *sessionRecords;
+    [self.sessionDBConnection readWriteWithBlock:^(YapDatabaseReadWriteTransaction *transaction) {
+        sessionRecords =
+            [transaction objectForKey:contactIdentifier inCollection:TSStorageManagerSessionStoreCollection];
+
+
+        for (id deviceId in sessionRecords) {
+            id object = sessionRecords[deviceId];
+            if (![object isKindOfClass:[SessionRecord class]]) {
+                OWSFail(@"Unexpected object in session dict: %@", object);
+                continue;
+            }
+
+            SessionRecord *sessionRecord = (SessionRecord *)object;
+            [sessionRecord archiveCurrentState];
+        }
+
+        [transaction setObject:sessionRecords
+                        forKey:contactIdentifier
+                  inCollection:TSStorageManagerSessionStoreCollection];
+    }];
+}
+
 #pragma mark - debug
 
 - (void)printAllSessions