[capwap] Implement reqeust queue
[capwap] Implement reqeust queue

Summary: It is allowd only one outstanding request for the wtp. All
         other requests will be send in a queue, until the first request
         will be acked.

Test Plan: read

Reviewers: aschultz, orixmann

CC: mkraemer, aschultz

Maniphest Tasks: T290

Differential Revision: https://review.tpip.net/D1256

travelping/capwap,travelping/capwap