Compat code for new wallet

This PR adds upgrade code for migrating from the old wallet.json to the new wallet.db.

Not visible in this PR is the merge commit (66bcadf5) I made to update the wallet branch to the latest master. So far I have not been able to get the branch to pass the TestRentAndHostReload api test. I would appreciate some help in tracking down why exactly the test fails. The error message is:

renterhost_test.go:1172: wallet has coins spent in incomplete transactions - not enough remaining coins

I suspected defragging was the cause, but disabling defragging does not fix the problem.