Implement addmultisigaddress and createmultisig.
rework the account manager somewhat.

- Move the MarkAddresForAccount and LookupAccountByAddress functionality
into account maanger.

- Move the wallet opeing logic into account manager (the only place that calls
it) and unexport.

- Move accountHandler to using a single channel for commands. Many of
the commands have ordering restraints (add account, list all accounts,
remove account, access account, mark account for address) which are very
much undefined with the multi-channel model.

- Rework all callers of LookupAccountByAddress to get the account structure
directly.