Batching/prebuilding next qt5 integration

To speed up and perhaps organize process of merging qt5 changes. 

Current problems come from:

• Qt5 integration is too long/resource intensive in total ~2900 VMs when creating this ticket. Length wise up to 12h.
• Too high failure rate. Health check metrics indicates that already merged qt5 has 5% chance to pass on first try when integrating. 

With those when changes get staged separately they might have different workitems sha1s meaning that results between them cannot be used. Additionally when some change merges it has possibility to invalidate everything that another change's stages have built.

This can lead to almost perpetual cycle where different sets of changes are staged and the progress is always reset. 

Some ideas

• Grace period after failure which allows rerun before changes are brought back to new state.
• Allow easier prebuilding/merge train of the changes. This requires mechanism where flakiness does not cause the system to reject the set.