Merge #15849: Thread names in logs and deadlock debug tools

8722e54e56 threads: add thread names to deadlock debugging message (James O'Beirne)
383b186c28 threads: prefix log messages with thread names (James O'Beirne)
ddd95ccb80 tests: add threadutil tests (James O'Beirne)
ae5f2b6a6c threads: introduce util/threadnames, refactor thread naming (James O'Beirne)
188ca75e5f disable HAVE_THREAD_LOCAL on unreliable platforms (James O'Beirne)

Pull request description:

  I'm resurrecting this one (from #13168) because I need it to make progress on #15735.

  It's now off by default and can be turned on with `-logthreadnames=1`.

  Ran some benchmarks (IBD from local peer from 500_000 -> 504_000) and it's within spitting distance either on or off:

  ### threadnames off (default)

  #### 2018-05-threadnames.3 vs. master (absolute)
  |                      name                      | iterations |   2018-05-threadnames.3    |           master           |
  |------------------------------------------------|-----------:|----------------------------|----------------------------|
  | ibd.local.500000.504000.dbcache=2048           |          3 | 376.1584 (± 9.2944)        | 392.3414 (± 13.4238)       |
  | ibd.local.500000.504000.dbcache=2048.mem-usage |          3 | 2236117.3333 (± 1845.9623) | 2238690.6667 (± 2669.3487) |

  #### 2018-05-threadnames.3 vs. master (relative)
  |                      name                      | iterations | 2018-05-threadnames.3 | master |
  |------------------------------------------------|-----------:|----------------------:|-------:|
  | ibd.local.500000.504000.dbcache=2048           |          3 |                     1 |  1.043 |
  | ibd.local.500000.504000.dbcache=2048.mem-usage |          3 |                     1 |  1.001 |

  ### threadnames on

  #### 2018-05-threadnames-take-2 vs. master (absolute)
  |                      name                      | iterations | 2018-05-threadnames-take-2 |           master           |
  |------------------------------------------------|-----------:|----------------------------|----------------------------|
  | ibd.local.500000.504000.dbcache=2048           |          3 | 367.6861 (± 0.3941)        | 364.1667 (± 0.9776)        |
  | ibd.local.500000.504000.dbcache=2048.mem-usage |          3 | 2238461.3333 (± 3697.8730) | 2237014.6667 (± 3307.6966) |

  #### 2018-05-threadnames-take-2 vs. master (relative)
  |                      name                      | iterations | 2018-05-threadnames-take-2 | master |
  |------------------------------------------------|-----------:|---------------------------:|-------:|
  | ibd.local.500000.504000.dbcache=2048           |          3 |                      1.010 |   1.00 |
  | ibd.local.500000.504000.dbcache=2048.mem-usage |          3 |                      1.001 |   1.00 |
  ```

ACKs for commit 8722e5:
  Empact:
    utACK https://github.com/bitcoin/bitcoin/pull/15849/commits/8722e54e56fd959fd4ff2321b36a7640dee440c5
  jnewbery:
    utACK 8722e54e56fd959fd4ff2321b36a7640dee440c5
  MarcoFalke:
    re-utACK 8722e54e56fd959fd4ff2321b36a7640dee440c5 (Only change since my previous review is DEFAULT_LOGTHREADNAMES=false and stylistic updates

Tree-SHA512: 50af992708295b8d680cf10025262dd964e599a356bdfc1dfc84fb18c00afabcb34d3d12d551b0677ff81f8fccad0e17c1d5b24dfecb953a913bc77fdd1a4577

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