make transaction timeout user configurable

[weyrick]

Currently TransactionManager supports a custom TTL in the constructor, but this is not exposed to end users.

For the DNS Handler, we should expose a new handler configuration variable xact_ttl_secs to set this.

https://github.com/ns1labs/pktvisor/blob/db7b32d1f400273f44a222768d89e622ae5f4fd8/libs/visor_transaction/TransactionManager.h#L60