Expand user session creation abilities

[chris-allan]

A system wide, configurable maximum time to idle and time to live is now in place. Regular users are allowed to create new sessions with timeouts up to these limits. The previous limits were 10x the default time to idle and time to live and those are now the default configured values.

Furthermore, it is now possible to pass an "omero.agent" as part of call context to set the user agent on sessions created using the session service. For example:

createUserSession(0, 600000, 'test', {'omero.agent': 'my agent'})

Finally, the following properties have been exposed to the via the configuration service so that a user may make predictable and informed choices during session creation:

• omero.sessions.timeout
• omero.sessions.maximum
• omero.sessions.max_user_time_to_idle
• omero.sessions.max_user_time_to_live

[chris-allan]

Requires ome/omero-server#79 and ome/omero-common#21 to function but should compile. Will likely cause runtime errors without the aforementioned PRs so might need to be excluded until they are reviewed and merged.

/cc @joshmoore, @jburel

[jburel]

The changes do not conflict with @mtbc's work