Pending transactions are not correctly expired

[nodestory]

A pending tx is supposed to be expired after 3 days. Several hundred outdated transactions are still circulating on livenet (v0.12.4).

[nodestory]

There is one node that is still running the older version of bitmarkd. After the node is shut down or upgraded, this issue could be resolved.

A proper way of expiring transactions is necessary. Including block_number (like the design in share transactions) could be an option.

[hxw]

the version 0.13.2 has code to try and mitigate this