6f53edb Acquire cs_main before ATMP call in block_assemble bench (James O'Beirne)

Pull request description:

  Calling `bench_bitcoin` currently fails due to calling ATMP without acquiring cs_main first in the recently added block_assemble bench (bitcoin#13219).

  ```
  $ cat <(uname -a) <(gcc --version)

  Linux james 4.4.0-119-generic bitcoin#143+jamesob SMP Mon Apr 16 21:47:24 EDT 2018 x86_64 x86_64 x86_64 GNU/Linux
  gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609

  $ ./src/bench/bench_bitcoin

  WARNING: This is a debug build - may result in slower benchmarks.
  # Benchmark, evals, iterations, total, min, max, median
  Assertion failed: lock cs_main not held in validation.cpp:566; locks held:
  [1]    19323 abort (core dumped)  ./src/bench/bench_bitcoin
  ```

  ```
  (gdb) bt
  #0  0x00007fbdc9cf5428 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:54
  #1  0x00007fbdc9cf702a in __GI_abort () at abort.c:89
  #2  0x0000555a19580dc5 in AssertLockHeldInternal (pszName=pszName@entry=0x555a19834549 "cs_main",
      pszFile=pszFile@entry=0x555a1988a001 "validation.cpp", nLine=nLine@entry=566, cs=cs@entry=0x555a19ba55c0 <cs_main>) at sync.cpp:157
  #3  0x0000555a194b395f in AcceptToMemoryPoolWorker (chainparams=..., pool=..., state=...,
      ptx=std::shared_ptr (count 1, weak 0) 0x555a1bb819b0, pfMissingInputs=pfMissingInputs@entry=0x0, nAcceptTime=1532964079,
      plTxnReplaced=0x0, bypass_limits=false, nAbsurdFee=@0x7ffcbc1719d8: 0, coins_to_uncache=std::vector of length 0, capacity 0,
      test_accept=false) at validation.cpp:566
  #4  0x0000555a194ba661 in AcceptToMemoryPoolWithTime (chainparams=..., pool=..., state=...,
      tx=std::shared_ptr (count 1, weak 0) 0x555a1bb819b0, pfMissingInputs=pfMissingInputs@entry=0x0, nAcceptTime=<optimized out>,
      plTxnReplaced=0x0, bypass_limits=false, nAbsurdFee=0, test_accept=false) at validation.cpp:998
  #5  0x0000555a194ba7ce in AcceptToMemoryPool (pool=..., state=..., tx=std::shared_ptr (count 1, weak 0) 0x555a1bb819b0,
      pfMissingInputs=pfMissingInputs@entry=0x0, plTxnReplaced=plTxnReplaced@entry=0x0, bypass_limits=bypass_limits@entry=false, nAbsurdFee=0,
      test_accept=false) at validation.cpp:1014
  #6  0x0000555a19363fbe in AssembleBlock (state=...) at bench/block_assemble.cpp:102
  #7  0x0000555a193654d3 in std::_Function_handler<void (benchmark::State&), void (*)(benchmark::State&)>::_M_invoke(std::_Any_data const&, benchmark::State&) (__functor=..., __args#0=...) at /usr/include/c++/5/functional:1871
  #8  0x0000555a193501d7 in std::function<void (benchmark::State&)>::operator()(benchmark::State&) const (this=this@entry=0x555a1ba2cda0,
      __args#0=...) at /usr/include/c++/5/functional:2267
  #9  0x0000555a1934ec4c in benchmark::BenchRunner::RunAll (printer=..., num_evals=5, scaling=<optimized out>, filter=..., is_list_only=false)
      at bench/bench.cpp:121
  #10 0x0000555a1934ade9 in main (argc=<optimized out>, argv=<optimized out>) at bench/bench_bitcoin.cpp:92
  ```

Tree-SHA512: fdd7b28ff123ccea7a4f334d53f735d0c0f94aa9cc52520c2dd34dca45d78c691af64efcd32366fc472fedffbd79591d2be2bb3bfc4a5186e8712b6b452d64e3