mu: optimize indexing (get_uid_term)

Some users were report seeing get_uid_term high in the profiles; so
optimize this:

- make mu_util_get_hash a static inline function (used by get_uid_term)
- don't use 'realpath' in get_uid_term, seem that's the main culprit
- some slight faster string handling there too.

lib/mu-store-priv.hh

@@ -177,7 +177,8 @@ public:
 			mu_contacts_clear (_contacts);
 	}
 
-	std::string get_uid_term (const char *path) const;
+	// not re-entrant; stays valid until called again
+	const char *get_uid_term (const char *path) const;
 
 	MuContacts* contacts() { return _contacts; }