Move keyframe and gaps management into kmsutils

Change-Id: I8128adfb725cc0c77959a012e6d119c72bb70f4e
Use gstreamer utility to create key_unit event

Change-Id: I8416e8ad6af23144cc895355ecc5a5d7acf6fd39
Improve keyframe event management

With this change we always request PLI in a better way

Change-Id: I8317025494af94072c47d820f7773d5c1591d073
keyframe management: request only all_headers when new connection

Change-Id: I9c659d0ae42f5677f2694714d9a241fb70286f4c
kmsagnosticbin: drop buffers until key frame when src pad is connected

Change-Id: Ib1e4b7d1f54d00b6a6606b2770ad0ef02aeebfc6
kmsagnosticbin: request key frame when decoder is added

Change-Id: I982ca2c3bb55b0a8ecef416c4cb9aee241d52893