lib/db: Keep folder meta data persistently in db (fixes #4400)

This keeps the data we need about sequence numbers and object counts
persistently in the database. The sizeTracker is expanded into a
metadataTracker than handled multiple folders, and the Counts struct is
made protobuf serializable. It gains a Sequence field to assist in
tracking that as well, and a collection of Counts become a CountsSet
(for serialization purposes).

The initial database scan is also a consistency check of the global
entries. This shouldn't strictly be necessary. Nonetheless I added a
created timestamp to the metadata and set a variable to compare against
that. When the time since the metadata creation is old enough, we drop
the metadata and rebuild from scratch like we used to, while also
consistency checking.

A new environment variable STCHECKDBEVERY can override this interval,
and for example be set to zero to force the check immediately.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4547
LGTM: imsodin

lib/db/structs.proto

@@ -37,3 +37,20 @@ message FileInfoTruncated {
     int64                 sequence       = 10;
     string                symlink_target = 17;
 }
+
+// For each folder and device we keep one of these to track the current
+// counts and sequence. We also keep one for the global state of the folder.
+message Counts {
+    int32 files       = 1;
+    int32 directories = 2;
+    int32 symlinks    = 3;
+    int32 deleted     = 4;
+    int64 bytes       = 5;
+    int64 sequence    = 6; // zero for the global state
+    bytes deviceID    = 17; // device ID for remote devices, or special values for local/global
+}
+
+message CountsSet {
+    repeated Counts counts  = 1  [(gogoproto.nullable) = false];
+    int64           created = 2; // unix nanos
+}