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

cmd/syncthing/main.go

@@ -197,6 +197,11 @@ are mostly useful for developers. Use with care.
                    "minio" for the github.com/minio/sha256-simd implementation,
                    and blank (the default) for auto detection.
 
+ STDBCHECKEVERY    Set to a time interval to override the default database
+                   check interval of 30 days (720h). The interval understands
+                   "h", "m" and "s" abbreviations for hours minutes and seconds.
+                   Valid values are like "720h", "30s", etc.
+
  GOMAXPROCS        Set the maximum number of CPU cores to use. Defaults to all
                    available CPU cores.