Merge #16805: logs: add timing information to FlushStateToDisk()

dcef9a2922 logs: add timing information to FlushStateToDisk() (James O'Beirne)
41edaf227a logs: add BCLog::Timer and related macros (James O'Beirne)

Pull request description:

  It's currently annoying to detect FlushStateToDisk() calls when benchmarking since they have to be inferred from a drop in coins count from the `UpdateTip: ` log messages. This adds a new logging utility, `BCLog::Timer`, and some related macros that are generally useful for printing timing-related logging messages, and a message that is unconditionally written when the coins cache is flushed to disk.

  ```
  2019-09-04T20:17:51Z FlushStateToDisk: write block and undo data to disk completed (3ms)
  2019-09-04T20:17:51Z FlushStateToDisk: write block index to disk completed (370ms)
  2019-09-04T20:17:51Z FlushStateToDisk: write coins cache to disk (2068451 coins, 294967kB) completed (21481ms)
  ```

ACKs for top commit:
  laanwj:
    Thanks, ACK dcef9a2922
  ryanofsky:
    Code review ACK dcef9a2922. No changes since last review other than moving code to new timer.h header

Tree-SHA512: 6d61e48a062d3edb48d0e056a6f0b1f8031773cc99289ee4544f8349d24526b88519e1e304009d56e428f1eaf76c857bf8e7e1c0b6873a6f270306accb5edc3d

src/Makefile.test.include

@@ -127,6 +127,7 @@ BITCOIN_TESTS =\
   test/key_io_tests.cpp \
   test/key_tests.cpp \
   test/limitedmap_tests.cpp \
+  test/logging_tests.cpp \
   test/dbwrapper_tests.cpp \
   test/validation_tests.cpp \
   test/mempool_tests.cpp \