lib/db, lib/model: Always use reasonable sized batches (fixes #2250, fixes #4112)

Harmonize how we use batches in the model, using ProtoSize() to judge
the actual weight of the entire batch instead of estimating. Use smaller
batches in the block map - I think we might have though that batch.Len()
in the leveldb was the batch size in bytes, but it's actually number of
operations.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4114

lib/db/blockmap.go

@@ -19,7 +19,7 @@ import (
 
 var blockFinder *BlockFinder
 
-const maxBatchSize = 256 << 10
+const maxBatchSize = 1000
 
 type BlockMap struct {
 	db     *Instance