udb: use an account row interface.

[dnldd]

This updates account row related functions to use the account row interface for more flexibility and in preparation of adding new account types. Serialization and deserialization have been reduced to a one step process instead of converting to the intermediate dbAccountRow type first.

[jrick]

Hmm, so this isn't quite what I had in mind when I was talking about changing fetchAccountInfo to not return a type specific to BIP0044 accounts. I was instead thinking that the call should return some sort of interface type, and that following code to either operates on the generic account interface, or type asserts it for more specific features (e.g. type asserting to access an xpub).

[dnldd]

        // The serialized BIP0044 account raw data format is:
	//   <acctType><rdlen><encpubkeylen><encpubkey><encprivkeylen>
	//   <encprivkey><lastusedext><lastusedint><lastretext>
	//   <lastretint><namelen><name>
	//
	// 1 byte acctType + 4 bytes raw data length + 4 bytes encrypted
	// pubkey len + encrypted pubkey + 4 bytes encrypted privkey len +
	// encrypted privkey + 4 bytes last used external index + 4 bytes
	// last used internal index + 4 bytes last returned external +
	// 4 bytes last returned internal + 4 bytes name len + name

rdlen seems unnecessary at this point, should there be an account row upgrade to remove it?