add push priority method for right push vs left push into list

[joshdurbin]

Thoughts on this?

The scenario I've come across where I need this is that there are situations where you might want to push a message to the prescribed push queue, but at the tail of the list rather than the head. An example is a consumer that might be long running, where the consumer takes a context and can be be interrupted. In such a case, we might ideally start re-processing that message sooner than other queued items in the list for the purposes of logs for certain types of messages being more or less sequential, etc.. etc.. This is definitely in the "nice to have" column for me and I'm curious what others think.