&log keyword, and vector logging.

The &log keyword now operates as discussed:

    - When associated with individual record fields, it defines them
      as being logged.

    - When associated with a complete record type, it defines all fields
      to be logged.

    - When associated with a record extension, it defines all added
      fields to be logged.

    Note that for nested record types, the inner fields must likewise
    be declared with &log. Consequently, conn_id is now declared with
    &log in bro.init.

Vectors are now allowed to be logged and will be recorded as an
ordered set of items.

src/Attr.h

@@ -35,6 +35,7 @@ typedef enum {
 	ATTR_MERGEABLE,
 	ATTR_PRIORITY,
 	ATTR_GROUP,
+	ATTR_LOG,
 	ATTR_TRACKED,	// hidden attribute, tracked by NotifierRegistry
 #define NUM_ATTRS (int(ATTR_TRACKED) + 1)
 } attr_tag;