API: Optionally include in job queue size in maxlag

API: Optionally include in job queue size in maxlag

maxlag is the default mechanism most bots and libraries use in
determining when to back off due to wiki overload. However these days,
there are other things that should be considered when asking bots to
back off, one of those is job queue size.

For compatibility and simplicity of use, the number of jobs is converted
into something resembling seconds using a configurable factor. We also
output the total number of jobs in the API error output so more
sophisticated clients can do a more advanced back off.

Bug: T160003
Change-Id: Iedae2344a3d93202efbdd1bf807cef6165b6257a