MINOR: dict: Add dictionary new data structure.

This patch adds minimalistic definitions to implement dictionary new data structure
which is an ebtree of ebpt_node structs with strings as keys. Note that this has nothing
to see with real dictionary data structure (maps of keys in association with values).
diff --git a/include/common/hathreads.h b/include/common/hathreads.h
index 0ba56d0..79a6466 100644
--- a/include/common/hathreads.h
+++ b/include/common/hathreads.h
@@ -547,6 +547,7 @@
 	TLSKEYS_REF_LOCK,
 	AUTH_LOCK,
 	LOGSRV_LOCK,
+	DICT_LOCK,
 	OTHER_LOCK,
 	LOCK_LABELS
 };
@@ -663,6 +664,7 @@
 	case TLSKEYS_REF_LOCK:     return "TLSKEYS_REF";
 	case AUTH_LOCK:            return "AUTH";
 	case LOGSRV_LOCK:          return "LOGSRV";
+	case DICT_LOCK:            return "DICT";
 	case OTHER_LOCK:           return "OTHER";
 	case LOCK_LABELS:          break; /* keep compiler happy */
 	};