hydra-queue-runner: Maintain count of active build steps

Dependencies

• [2] ZSBPKI2P Pass ssh key
• [3] PQFOMNTL hydra-queue-runner: More stats
• [*] 5AIYUMTB Basic remote building
• [*] 24BMQDZA Start of single-process hydra-queue-runner
• [*] NJJ7H64S Very basic multi-threaded queue runner
• [*] OCZ4LSGG Automatically retry aborted builds
• [*] RQUAATWB Add status dump facility

Change contents

• edit in src/hydra-queue-runner/build-remote.cc at line 42
  [5.917]

  [2.0]
  + // FIXME: ensure no password prompt.

  + // FIXME: connection timeouts
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 42
  [7.908]

  [7.908]
  + struct MaintainCount

  + {

  + counter & c;

  + MaintainCount(counter & c) : c(c) { c++; }

  + ~MaintainCount() { c--; }

  + };
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 49
  [7.909]

  [6.5060]
  +
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 239
  [3.132]

  [3.132]
  + counter nrActiveSteps{0};
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 968
  [8.2749]

  [5.12653]
  + MaintainCount mc(nrActiveSteps);

  +
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 1265
  [9.934]

  [3.431]
  + printMsg(lvlError, format("%1% active build steps") % nrActiveSteps);