Umbrella ticket for multi-block election tasks to improve after PR#6034

[gpestana]

These tasks are part of the feedback from the #6034 review, they should be closed before multi-block is used in Polkadot and Kusama.

• Election miner implementation should be able to trim an election result to fit into the expected bounds #6237
  • implemented in Adds max backers per winner bounds check in the phragmen implementation #6482, under review
• Efficient upsert of exposures in multi-paged election #6499
• Add sort pause in bags list and trait SortedListProvider #6328
  • wip in Implements a mechanism to lock/unlock the SortedListProvider and bags-list iter #6510
• [EPM-MB] Consider refactoring the ElectionDataProvider and ElectionProvider traits and remove self-weighting in key methods #6136