clean up messagehandler a bit

remove unused cruft. rename registerSender function to
createRegisterMessage to reflect that it really only creates that message.

Jira-issue: WP-850
more messagehandler cleanup

use strings for setting routes by fixing how Array.join is used. rename ownId
to ownSessionId, including the setter and getter.

Jira-issue: WP-850
remove duplicated code in messagehandler

use function for getting the PZH sesssion id

Jira-issue: WP-850
drop msgs if local wrt disconnected

if received msg is intended for wrt connected to this pzp, do not forward msg
back to pzh. fixes infinite msg loop.

Jira-issue: WP-850
Merge pull request #525 from aldafu/WP-850-fix-messagehandler-infinite-msg-loop

Wp 850 fix messagehandler infinite msg loop

Tested and works.