bundle2: part handler for processing .hgtags fnodes mappings
.hgtags fnodes cache entries can be expensive to compute, especially
if there are hundreds of even thousands of them. This patch implements
support for receiving a bundle2 part that contains a mapping of
changeset to .hgtags fnodes.
An upcoming patch will teach the server to send this part, allowing
clients to bypass having to redundantly compute these values.
A number of tests changed due to the client advertising the "hgtagsfnodes"
capability.