Provide benchmark method

[jmoore]

Something like "bin/omero benchmark" would be useful for showing where a system may be less fast than desirable. Individual sections could include:

• IO (reading writing randomly to disk)
• NET (reading randomly from server; only once per session?)
• OMERO (reading some large list; eventlogs?)

The NET test could also be used on login to determine the optimal bandwidth.

cF. pgbench results:

transaction type: TPC-B (sort of)
scaling factor: 10
query mode: simple
number of clients: 10
number of transactions per client: 1000
number of transactions actually processed: 10000/10000
tps = 85.184871 (including connections establishing)
tps = 85.296346 (excluding connections establishing)