contacts: rework: install in database

Rewrite the contacts-cache backend in c++

Store the contacts as metadata in the xapian database, rather than in a
separate file.

Update the Store to deal with this.

lib/mu-store.h

@@ -24,6 +24,7 @@
 #include <inttypes.h>
 #include <mu-msg.h>
 #include <mu-util.h> /* for MuError, MuError */
+#include <mu-contacts.h>
 
 G_BEGIN_DECLS
 
@@ -146,6 +147,16 @@ void  mu_store_set_batch_size (MuStore *store, guint batchsize);
 void mu_store_set_my_addresses (MuStore *store, const char **my_addresses);
 
 
+/**
+ * Get the a MuContacts* ptr for this store.
+ *
+ * @param store a store
+ *
+ * @return the contacts ptr
+ */
+MuContacts* mu_store_contacts (MuStore *store);
+
+
 /**
  * get the numbers of documents in the database
  *