Account gap check works incorrectly after 100 accounts

[jrick]

The comment on Wallet.NextAccount currently says:

// NextAccount creates the next account and returns its account number.  The
// name must be unique to the account.  In order to support automatic seed
// restoring, new accounts may not be created when all of the previous 100
// accounts have no transaction history (this is a deviation from the BIP0044
// spec, which allows no unused account gaps).

However it does not behave this way. Instead, if the account number is over 100, it only checks if the previous account has any history, instead of checking the last 100.

The account has history check is also broken. It is intended to check whether there has been any transaction activity for the account, but instead it only checks if addresses have been generated for the account.