Make seq num an option on tx

[leighmcculloch]

We don't have to do this in this PR, but we should make sequence number an option, that if isn't set uses the nxt seq num lookup, otherwise uses the value passed in. A good follow up to add later.

Originally posted by @leighmcculloch in #1551 (comment)

cc @willemneal

[leighmcculloch]

After playing with this new option, I think we should change the design.

The new option is not really usable where it is in a meaningful way without being coupled with --build-only. The reason being is that any use where the tx will be sent immediately really just needs to use the current sequence number plus one. The only need for setting a different sequence number occurs when building and later sending.

Since the option is only usable in that advanced build-only case, I think we should make this something they can do with the stellar tx ... commands. It will keep the options advertised smaller for each of the subcommands being changed here, and keeps the advanced functionality for tx modifications in the same place. Maybe as a way to simply change a sequence number stellar tx set seq, where those commands could grow to support others like stellar tx set source-account. Or maybe by using the stellar tx edit.

Originally posted by @leighmcculloch in #1702 (comment)