Fix race in finishing builds that have been cancelled

Dependencies

• [2] T5BIOVJE Add support for tracking custom metrics
• [*] 24BMQDZA Start of single-process hydra-queue-runner
• [*] HPJKBFZ4 Handle concurrent finishing of the same build
• [*] 5AIYUMTB Basic remote building
• [*] 62MQPRXC Pass null values to libpqxx properly

Change contents

• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 213
  [5.1708]

  [6.14683]
  +

  + if (txn.parameterized("select 1 from Builds where id = $1 and finished = 0")(build->id).exec().empty()) return;
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 226
  [7.716]

  [4.18276]
  +

  + txn.parameterized("delete from BuildProducts where build = $1")(build->id).exec();
• edit in src/hydra-queue-runner/hydra-queue-runner.cc at line 245
  [4.19015]

  [2.1332]
  + txn.parameterized("delete from BuildMetrics where build = $1")(build->id).exec();

  +