Use hashtable when parsing a message

Use hashtable when parsing a message

When parsing messages use a hashtable instead of a linear search to
reduce the amount of work done in findname when there's more than one
name in the section.

There are two hashtables:

1) hashtable for owner names - that's constructed for each section when
we hit the second name in the section and destroyed right after parsing
that section;

2) per-name hashtable - for each name in the section, we construct a new
hashtable for that name if there are more than one rdataset for that
particular name.

(cherry picked from commit b8a96317544c7b310b4f74360825a87b6402ddc2)