server: implement postgres queue driver about community

[plyr4]

i see two options:

1. we could try to tap into the existing postgres database driver and add a builds_queue "service", like adding a new table for something else.
2. we implement another driver under the queue package and keep queuing code completely separate from the standard database code.

for 1 there would be less duplicated code, and the server could re-use the existing connection to the primary database. however, it might be more complex.

perhaps there is a middle ground where the queue package could implement the functionality, but it allows for an existing database configuration to be passed in. this might make it more difficult to customize queue read timeouts, etc.

im leaning towards a separate driver (2)

[plyr4]

if there's no activity or buy in for this i'll just close it when it gets stale