Allow different account type.

This commit modifies the account serialization and fetching in order to
support multiple account types, even though there is currently only
support for BIP0044 accounts.

As a part of this, the serialization and deserialization code has been
changed to use regular key/value instead of an individual bucket per
account.  This was done because bolt specifically recommends against using
a lot of small buckets to avoid poor page utilization once they grow past
a certain size.  While the current account type does not hit that
threshold, other account types might, so just change it over now.