Mesh 1967 cleanup checkpoint schedules

[Neopallium]

changelog

new features

• Simplify Checkpoint schedules. Schedules are now just a list of pending Moment checkpoints, instead of complex calendar basic repeating values.

modified API

• Checkpoint.create_schedule now take a ScheduleCheckpoints (holds a BTreeSet of Moment values) instead of ScheduleSpec.
• Events ScheduleCreated and ScheduleRemoved changed to have ScheduleId and ScheduleCheckpoints instead of StoredSchedule.
• Storage map Schedules replaced with double map ScheduledCheckpoints. Schedules are no longer stored in a Vec.
• Storage map CachedNextCheckpoints is used to quickly check if a ticker has any checkpoints that need to be created.

modified logic

• No longer supports recurring schedules. All scheduled checkpoints timestamps (Moment) need to be calculated off-chain.

data migration

• Non-empty schedules from Checkpoint.Schedules are migrated to new structure, only a maximum of the next 10 checkpoints are kept for each schedule.

[codecov]

Codecov Report

Merging #1488 (9ac117c) into develop (0fadd64) will decrease coverage by 0.23%.
The diff coverage is 67.04%.

@@             Coverage Diff             @@
##           develop    #1488      +/-   ##
===========================================
- Coverage    54.43%   54.21%   -0.23%     
===========================================
  Files          151      151              
  Lines        32960    32988      +28     
===========================================
- Hits         17942    17884      -58     
- Misses       15018    15104      +86     

Impacted Files	Coverage Δ
pallets/asset/src/lib.rs	81.78% <ø> (+0.04%)	⬆️
pallets/runtime/common/src/runtime.rs	1.81% <ø> (ø)
pallets/weights/src/pallet_checkpoint.rs	0.00% <0.00%> (ø)
primitives/src/calendar.rs	36.30% <ø> (-37.36%)	⬇️
pallets/asset/src/checkpoint/mod.rs	45.47% <58.37%> (-12.42%)	⬇️
pallets/corporate-actions/src/lib.rs	39.14% <84.21%> (+0.39%)	⬆️
pallets/common/src/traits/checkpoint.rs	74.82% <85.24%> (+61.03%)	⬆️
primitives/src/asset.rs	49.05% <100.00%> (+3.05%)	⬆️

... and 3 files with indirect coverage changes