Retrieve more info about jobs when using 'at' adaptor

[jmaassen]

Currently the at adaptor returns very little info once a job has finished. Even getting the correct state or exit code is hard (see #683).

We could get a bit more information about the job by using an external tool such as '/bin/time like so:

/bin/time -v xenon filesystem file list /home/jason > out 
	Command being timed: "xenon filesystem file list /home/jason"
	User time (seconds): 1.15
	System time (seconds): 0.05
	Percent of CPU this job got: 259%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.46
	Average shared text size (kbytes): 0
	Average unshared data size (kbytes): 0
	Average stack size (kbytes): 0
	Average total size (kbytes): 0
	Maximum resident set size (kbytes): 87552
	Average resident set size (kbytes): 0
	Major (requiring I/O) page faults: 0
	Minor (reclaiming a frame) page faults: 18320
	Voluntary context switches: 1334
	Involuntary context switches: 233
	Swaps: 0
	File system inputs: 0
	File system outputs: 80
	Socket messages sent: 0
	Socket messages received: 0
	Signals delivered: 0
	Page size (bytes): 4096
	Exit status: 0

Time provides information on resource usage, as well as the exit code, but only once the application has finished. We could use this to write a /tmp/xenon.at.finished.#ID file which the at adaptor can use to get information about finished jobs.