lib/config: set default batch-size to 50000

The default was 250000 but that led to problems on some systems with
limited memory, esp. since mu's indexing does quite a bit more than in
the olden days (e.g. html mail).

Fixes #2529.

lib/mu-config.hh

@@ -100,8 +100,8 @@ public:
 			Type::Number,
 			Flags::Configurable,
 			"batch-size",
-			"250000",
+			"50000",
-			"Version of the Xapian database schema"
+			"Number of changes in a database transaction"
 		},
 		{
 			Id::Contacts,