Create a set of subclasses to ensure we can't forget prefixes or create
inconsistent ones.

This change exactly matches the previous output, I want to make sure it's
clear where I've changed the output vs. where I'm changing how we create
that output.

Reference #1313.
Use the "btree" tag consistently (before we had both "Btree" and "btree").
Replace some of the tags with more verbose versions:

    conn -> connection,
    dhandle -> data-handle
    txn -> transaction

Don't use whitespace in the tag name so post-processing is a little
simpler:

    block manager -> block-manager
whitespace
Make capitalization consistent.
Make capitalization consistent.
Now that the tag is part of the description, the right field to sort
on is the description, not the name.
Get rid of some abbreviations in the async statistics messages.
Update the test suite to match the new tag names.
Merge pull request #1338 from wiredtiger/stat-tags

Statistics review/cleanup.