Implements _reindex_shares

Since shares are ints, a bidirectional directionary of ints to ints
creates conflicts. Therefore when shares are reindexed, two dicts
must be created, one with shares -> indexes and one with indexes
-> shares.

Also fixes a small issue when a set was incorrectly being added
to existing_peers.
Implements PeerSelector into Tahoe2ServerSelector

Initial attempt to refactor Tahoe2ServerSelector in terms of
the PeerSelector. This does not pass all of the existing
tests in test_upload.py