HPCC-12128 - MP connections take too long

When A-> B and B->A within a very short time frame a 3 minute delay would often occur after which a failed connection might still occur.  This was noticed very often when using the mptests, which are used for new mp lib development.

Signed-off-by: Mark Kelly <mark.kelly@lexisnexis.com>
If you need to ensure dec on exit to attachSocket, would be cleaner to replace these with something like: 
struct dTor
{
    ~dTor() { atomic_dec(&attachchk); }
} attachChkDTor;

.. at the top of the method.