Stake tracker improvements (migration and try-state checks OK in Polkadot)

[gpestana]

Changes in the migration code introduced in this PR:

• Migration removes duplicate nominations in all nominators, if they exist (changes by calling fn do_add_nominator with dedup nominations)
• Migration removes all the non active validator nominations to avoid adding dangling nominations (changes by calling fn do_add_nominator if necessary)
• Migration iterates through all Nominators map first and through all the Validators map to insert all the validators that are not nominated to the target list (with self-stake only).
• Moves stake-tracker related try-state checks to the staking pallet (no try-state checks in the stake-tracker pallet anymore).
• Runs benchmarks for MMB migration step with bench bot.

The migration code has been validated against the Polkadot using the externalities tests in polkadot/runtime/westend/src/lib.rs. Upon running the migrations, we ensure that:

• All validators have been added to the target list with their correct approvals score (as per the try-state checks).
• All nominations are "cleaned" (see def. of clean above)
• Try-state checks related to stake-tracker and approvals pass.

Note: Same as #4436 but with a non-messed up git history. Merging into gpestana/stake-tracker_integration.