lib/mu-store: rework implementation

* mu-store.h, mu-store-read.cc, mu-store-write.cc, mu-store-priv.hh have been reworked
   in mu-store.{cc,hh}, it the mix of c/c++ improved
 * update all the dependent modules
 * make it easier to upgrade an database in place (without user intervention)
 * remove the xbatch-size option

lib/mu-index.h

@@ -25,7 +25,7 @@
 #include <stdlib.h>
 #include <glib.h>
 #include <mu-util.h> /* for MuResult */
-#include <mu-store.h>
+#include <mu-store.hh>
 
 G_BEGIN_DECLS
 
@@ -77,16 +77,6 @@ void mu_index_destroy (MuIndex *index);
 void mu_index_set_max_msg_size (MuIndex *index, guint max_size);
 
 
-/**
- * change batch size for Xapian store transaction (see
- * 'mu_store_set_batch_size')
- *
- * @param index a mu index object
- * @param max_size the batch size, or 0 to reset to the default
- */
-void mu_index_set_xbatch_size (MuIndex *index, guint xbatchsize);
-
-
 /**
  * callback function for mu_index_(run|stats|cleanup), for each message
  *