Align shutdown timestamp to draw period

[trmid]

The TWAB shutdown time is not guaranteed to be aligned with the prize pool draws. Since the prize pool checks shutdown status based on timestamps, it's possible that the prize pool can award a draw, have some prizes claimed, and then shutdown mid-draw. This causes issues with the shutdown portion calculation if any prizes were claimed in that time period (see original issue for more details).

To mitigate this, the TWAB shutdown timestamp is truncated to the draw period such that the prize pool is always guaranteed to be shutdown at the start of a draw.

[linear]

GEN-1775 [133] force align prize pool shutdown time to start of draw

[github-actions]

LCOV of commit 6d57310 during Tests with 100% Coverage #560

Summary coverage rate:
  lines......: 99.8% (519 of 520 lines)
  functions..: 100.0% (80 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| 100%    46|    -      0

[asselstine]

Nice