background transaction interface

[coburncoburn]

it is possible to record background transactions by setting the transaction key as shown below.

Opening this PR to get feedback on how to best expose this interface.

This could also be accomplished be done by simply removing the is_binary guard clause here and adding documentation about how to call Collector directly as here, but an interface on the transaction module may be nice.

The data will show up in the background jobs report.

[coburncoburn]

@britto what do you think of this?

[fbjork]

@coburncoburn Nice!!