Use Session.useragent for identifying active sessionss

[jmoore]

Moving into the current iteration. While working on #1704, it was easy to add support for agents. Currently, it is necessary to use the implicit context:

  client = omero.client()
  client.getImplicitContext().put(omero.constants.AGENT, "OMERO.web")
  sf = client.createSession();

Methods should be added to the omero.client object to set this value more simply. AND we need to decide if we want more intelligent agent strings than just "OMERO.web", "OMERO.insight", etc.

[jmoore]

(In [6458]) see #1974 - server-side changse for setting Session.userAgent

Note: unfortunately, some of the internal server changes for this commit made their way into the previous two commits.

[jmoore]

(In [6459]) see #1974 - Adding setAgent to all SDKs with sensible defaults

[jmoore]

(In [6460]) fix #1974 - using setAgent almost everywhere

Work is still needed to pass the agent setting down to BlitzGateway from OmeroWeb, and Insight will need a new jar before the agent can be set.