SSP-858 Allow KeepSessionAliveFilter to be disabled

Using Spring profiles, KeepSessionAliveFilter is now completely
disabled (not even on the call stack) when running in standalone
mode.

When running in uPortal, that filter can now be disabled via
ssp-config.properties:

  uportal_session_keep_alive_timeout=-1

Or you can go in completely the other direction and cause it
to run on every request:

  uportal_session_keep_alive_timeout=0

Default is 10s:

  uportal_session_keep_alive_timeout=10