runtime: Avoid redundant collections of stake delegations into a vector

[vadorovsky]

Problem

Stake delegations are stored in an im::HashMap, which is convenient to use as a per-bank cache, because of its CoW properties:

• clone() is just copying a pointer and increasing a reference counter
• post-clone modification of a single node, makes a copy only of that node, not of the whole map

However, im::HashMap:

• has terrible traversal performance - a full iteration over a map with 1,000,000 elements takes 200ms
• doesn't support parallel iteration with Rayon
  • using par_bridge performs even worse than a single-threaded iteration - it takes 1.3s

Therefore, it's better to collect() the im::HashMap before calculating rewards.

The problem is that there were two collect() calls made during one epoch boundary.

Summary of Changes

Fix that by collecting only once and passing the vector of delegations along.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.1%. Comparing base (4a3e05a) to head (925ca56).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7770   +/-   ##
=======================================
  Coverage    83.0%    83.1%           
=======================================
  Files         812      812           
  Lines      356900   356981   +81     
=======================================
+ Hits       296578   296658   +80     
- Misses      60322    60323    +1     

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[vadorovsky]

Closing in favor of #8065