Possible DoS for Stellar withdrawals

[tamirms]

I realized that sourceAccount.LastModifiedLedger is updated also when the account receives the deposit not only when the sequence is bumped. With the current single condition I think it's possible to block the withdrawal of any account by sending 1 stroop every ledger. I wonder how and if we can solve this.

EDIT: Looks like we could use CAP-21 and it's seqLedger which equals to "ledger number at which seqNum took on its present value". In Horizon it's Account.SequenceLedger but can be empty if not set in extension.

Originally posted by @bartekn in #68 (comment)

[bartekn]

Probably solved by 90255c8 in #68.