Allow multiple in-flight transactions for an account.

[hackaugusto]

Currently for simplicity only one transaction can be in-flight per account.

[hackaugusto]

Btw, what this the reason for this constraint? Is it just because the code is simpler? Or do we actually want to limit the concurrent in-flight transaction to 1 in the beginning? (I'm asking because it seems to lift this is pretty straightforward)

[bobbinth]

Btw, what is the reason for this constraint? Is it just because the code is simpler?

Yep - just made the code a bit simpler at the time when there were a lot of moving parts.

[bobbinth]

Some parts of discussions in #191 and #196 may be relevant here.

[bobbinth]

Also, #235 could also be good background info.

[bobbinth]

Closed #407.