Fix regression in server-side WDBS after 8c3c657.

Servers that support WDBS sessions should include the session ID in
its returned capabilities with the key webdriver.remote.sessionid. This
is relied on by (at least) the Ruby and Python RC clients.

This introduces some duplication between the NewSession and GetCapabilities
handlers; someone with better knowledge of the Java server may look into
moving this logic elsewhere.