hydra-queue-runner: Bump memory limit to reflect more accurate accounting

Dependencies

• [2] SJQC2I3N Bump memory limit a bit
• [3] 24BMQDZA Start of single-process hydra-queue-runner
• [4] CNLNT3T4 Allow only 1 thread to send a closure to a given machine at the same time
• [5] GJV2J5HX Pool local store connections
• [6] ENXUSMSV Make concurrency more robust
• [7] B2L4T3X6 Sync with Nix
• [8] HJOEIMLR Refactor
• [9] SL3WSRAC hydra-queue-runner: Limit memory usage

Change contents

• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 21
  [3.7501]→[2.0:64](∅→∅)
  − : memoryTokens(6ULL << 30) // FIXME: make this configurable
  [3.7501]

  [3.7501]
  + : memoryTokens(15ULL << 30) // FIXME: make this configurable