Update cost-tracker to return more updated costs after successful try_add call

[tao-stones]

Problem

After successful try_add(...) call, it currently return the updated block cost onlyu; It'd be useful to also include updated costliest account cost in the return, so the call site can know after adding TransactionCost to cost track, which account it updated has highest units.

One particular use case for that additional information is: when Forwarder decides which "batch" a forwardable packet should be placed into, it can look at its "costliest account cost", mod by each batch's account limit, to get the target batch index.

Summary of Changes

• add return struct UpdatedCosts
• updated related functions to return Result<UpdatedCosts> instead of Result<u64>

Fixes #

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 82.1%. Comparing base (12d009e) to head (6147400).

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #1141     +/-   ##
=========================================
- Coverage    82.1%    82.1%   -0.1%     
=========================================
  Files         881      881             
  Lines      235742   235749      +7     
=========================================
+ Hits       193677   193678      +1     
- Misses      42065    42071      +6