Reimplement address pools.

[jrick]

This PR is for a series of two commits and should not be squashed. The first commit adds a test artifact using the old version of the udb package. Code to generate the test database was included in this commit as well, and can be run again if necessary by checking out this commit. The second commit implements the upgrade and updates the wallet for the new APIs. The old test db code generation will not run on anything but the first commit and should not be updated.

The new implementation is much simpler and safer, and provides more
useful features as well. Addresses are now derived based on the last
used address, rather than incrementing and keeping track of a child
index to next use. Address activity lookahead has been implemented so
that other wallets sharing the same seed will stay in sync so long as
all wallets respect the BIP0044 gap limit.

Fix account generation after 100 accounts have been derived so that
derivation is allowed when any of the previous 100 accounts have
public activity rather than just the previous account.

This change involves a database upgrade to version 4. This version
removes the next-to-use account branch child indexes, replacing them
with the last used child index.

Fixes #112.
Fixes #411.
Fixes #580.
Fixes #603.
Fixes #635.

[jrick]

Going to need to get travis ignoring that wallet/udb/testdata directory:

wallet/udb/testdata/v1.db.go:111:15:warning: assignment count mismatch (2 vs 1) (gosimple)

[alexlyp]

code looks great, tACK on simnet getting hammered with ticketpurchases and sends