Freeze observations after shutdown

[trmid]

See description of issue here: #111 (comment)

[github-actions]

LCOV of commit 847263a during Tests with 100% Coverage #531

Summary coverage rate:
  lines......: 100.0% (518 of 518 lines)
  functions..: 97.4% (76 of 78 functions)
  branches...: no data found

Files changed coverage rate:
                                             |Lines       |Functions  |Branches    
  Filename                                   |Rate     Num|Rate    Num|Rate     Num
  =================================================================================
  src/PrizePool.sol                          | 100%    230|95.5%    44|    -      0

[github-actions]

LCOV of commit aea00c7 during Tests with 100% Coverage #533

Summary coverage rate:
  lines......: 100.0% (518 of 518 lines)
  functions..: 97.4% (76 of 78 functions)
  branches...: no data found

Files changed coverage rate:
                                             |Lines       |Functions  |Branches    
  Filename                                   |Rate     Num|Rate    Num|Rate     Num
  =================================================================================
  src/PrizePool.sol                          | 100%    230|95.5%    44|    -      0

[asselstine]

I think locking the draw id makes a ton of sense! This is the right approach.

If we don't update the vault accumulator then other functions will break: getContributedBetween, getVaultPortion, etc. We don't know how those functions will be used; so ideally we can keep them working as long as possible.

Instead I think we should insert the locking logic into the getOpenDrawId function, because that will have a broader trickle-down effect. Anything bound to getOpenDrawId will still be "correct", and the other functions will still work.

[github-actions]

LCOV of commit 060de4a during Tests with 100% Coverage #535

Summary coverage rate:
  lines......: 99.8% (519 of 520 lines)
  functions..: 97.5% (78 of 80 functions)
  branches...: no data found

Files changed coverage rate:
                                             |Lines       |Functions  |Branches    
  Filename                                   |Rate     Num|Rate    Num|Rate     Num
  =================================================================================
  src/PrizePool.sol                          |99.6%    232|95.7%    46|    -      0
  src/libraries/DrawAccumulatorLib.sol       | 100%     61| 100%     5|    -      0

⛔ The code coverage is too low. Expected at least 100.