Merge #11035: [contrib] Add Valgrind suppressions file

Merge #11035: [contrib] Add Valgrind suppressions file

4a426d8 Add note about Valgrind suppressions file in developer-notes.md (practicalswift)
84e2462 contrib: Add Valgrind suppressions file (practicalswift)

Pull request description:

  Includes known Valgrind warnings in our dependencies that cannot be fixed in-tree.

  Example use:

  ```
  $ valgrind --suppressions=contrib/valgrind.supp src/test/test_bitcoin
  $ valgrind --suppressions=contrib/valgrind.supp --leak-check=full \
        --show-leak-kinds=all src/test/test_bitcoin --log_level=test_suite
  ```

  Running with the suppressions file under Ubuntu 16.04:

  ```
  $ valgrind --suppressions=contrib/valgrind.supp --leak-check=full --show-leak-kinds=all src/test/test_bitcoin --log_level=test_suite --run_test=wallet_crypto
  …
  ==10769== LEAK SUMMARY:
  ==10769==    definitely lost: 0 bytes in 0 blocks
  ==10769==    indirectly lost: 0 bytes in 0 blocks
  ==10769==      possibly lost: 0 bytes in 0 blocks
  ==10769==    still reachable: 0 bytes in 0 blocks
  ==10769==         suppressed: 72,704 bytes in 1 blocks
  ```

  Running without the suppressions file under Ubuntu 16.04:

  ```
  $ valgrind --leak-check=full --show-leak-kinds=all src/test/test_bitcoin --log_level=test_suite --run_test=wallet_crypto
  …
  ==10724== 72,704 bytes in 1 blocks are still reachable in loss record 1 of 1
  ==10724==    at 0x4C2DBF6: malloc (vg_replace_malloc.c:299)
  ==10724==    by 0x6F74EFF: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21)
  ==10724==    by 0x40106B9: call_init.part.0 (dl-init.c:72)
  ==10724==    by 0x40107CA: call_init (dl-init.c:30)
  ==10724==    by 0x40107CA: _dl_init (dl-init.c:120)
  ==10724==    by 0x4000C69: ??? (in /lib/x86_64-linux-gnu/ld-2.23.so)
  ==10724==    by 0x2: ???
  ==10724==    by 0x1FFF0006D2: ???
  ==10724==    by 0x1FFF0006E8: ???
  ==10724==    by 0x1FFF0006FF: ???
  ==10724==
  ==10724== LEAK SUMMARY:
  ==10724==    definitely lost: 0 bytes in 0 blocks
  ==10724==    indirectly lost: 0 bytes in 0 blocks
  ==10724==      possibly lost: 0 bytes in 0 blocks
  ==10724==    still reachable: 72,704 bytes in 1 blocks
  ==10724==         suppressed: 0 bytes in 0 blocks
  ```

Tree-SHA512: 9c92079fc61313ea678deb6aaa16a3a71c3154c757459793eb9ca0d90a9a74c6faebfb04c9135e1b398ca34224fb7f03bd9c488ea0e8debf6894f69f030a31d3