Don't wait forever to acquire the send lock

Dependencies

• [2] BYVRA54Q Temporarily disable machines on any exception, not just connection failures
• [3] CNLNT3T4 Allow only 1 thread to send a closure to a given machine at the same time
• [4] 5AIYUMTB Basic remote building
• [5] FITVNQ2S Keep track of the time we spend copying to/from build machines
• [6] YR2IM6Y5 Temporarily disable machines after a connection failure
• [7] HJOEIMLR Refactor

Change contents

• replacement in src/hydra-queue-runner/build-remote.cc at line 232
  [2.2711]→[2.2711:2787](∅→∅)
  − std::lock_guard<std::mutex> sendLock(machine->state->sendLock);
  [2.2711]

  [2.2787]
  + std::unique_lock<std::timed_mutex> sendLock(

  + machine->state->sendLock, std::chrono::seconds(600));
• replacement in src/hydra-queue-runner/state.hh at line 234
  [3.286]→[3.286:315](∅→∅)
  − std::mutex sendLock;
  [3.286]

  [3.3777]
  + std::timed_mutex sendLock;