…mance with perf

13782b8 docs: add perf section to developer docs (James O'Beirne)
58180b5 tests: add utility to easily profile node performance with perf (James O'Beirne)

Pull request description:

  Adds a context manager to easily (and selectively) profile node performance during functional test execution using `perf`.

  While writing some tests, I encountered some odd bitcoind slowness. I wrote up a utility (`TestNode.profile_with_perf`) that generates performance diagnostics for a node by running `perf` during the execution of a particular region of test code.

  `perf` usage is detailed in the excellent (and sadly unmerged) bitcoin#12649; all due props to @eklitzke.

  ### Example

  ```python
  with node.profile_with_perf("large-msgs"):
      for i in range(200):
          node.p2p.send_message(some_large_msg)
      node.p2p.sync_with_ping()
  ```

  This generates a perf data file in the test node's datadir (`/tmp/testtxmpod0y/node0/node-0-TestName-large-msgs.perf.data`).

  Running `perf report` generates nice output about where the node spent most of its time while running that part of the test:

  ```bash
  $ perf report -i /tmp/testtxmpod0y/node0/node-0-TestName-large-msgs.perf.data --stdio \
    | c++filt \
    | less

  # To display the perf.data header info, please use --header/--header-only options.
  #
  #
  # Total Lost Samples: 0
  #
  # Samples: 135  of event 'cycles:pp'
  # Event count (approx.): 1458205679493582
  #
  # Children      Self  Command          Shared Object        Symbol
  # ........  ........  ...............  ...................  ........................................................................................................................................................................................................................................................................
  #
      70.14%     0.00%  bitcoin-net      bitcoind             [.] CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)
                  |
                  ---CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)

      70.14%     0.00%  bitcoin-net      bitcoind             [.] CNetMessage::readData(char const*, unsigned int)
                  |
                  ---CNetMessage::readData(char const*, unsigned int)
                     CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)

      35.52%     0.00%  bitcoin-net      bitcoind             [.] std::vector<char, zero_after_free_allocator<char> >::_M_fill_insert(__gnu_cxx::__normal_iterator<char*, std::vector<char, zero_after_free_allocator<char> > >, unsigned long, char const&)
                  |
                  ---std::vector<char, zero_after_free_allocator<char> >::_M_fill_insert(__gnu_cxx::__normal_iterator<char*, std::vector<char, zero_after_free_allocator<char> > >, unsigned long, char const&)
                     CNetMessage::readData(char const*, unsigned int)
                     CNode::ReceiveMsgBytes(char const*, unsigned int, bool&)

  ...
  ```

Tree-SHA512: 9ac4ceaa88818d5eca00994e8e3c8ad42ae019550d6583972a0a4f7b0c4f61032e3d0c476b4ae58756bc5eb8f8015a19a7fc26c095bd588f31d49a37ed0c6b3e