Backport r1593614 from trunk:

    Option to detect concurrent accesses to pools
    
    Enabled by new configure option --enable-pool-concurrency-check.
    
    Compared to pool-owner-debugging, this only detects cases where there is actual
    contention between accesses. The advantage is that runtime costs should be
    relatively low.
    
    The diagnostic messages could still use some improvement.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.6.x@1594707 13f79535-47bb-0310-9956-ffa450edef68