HPCC-11522 libmemcached plugin

Previously: #6368 & #6686

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
Changes from review

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
Final alterations
Signed-off-by: jamienoss <james.noss@lexisnexis.com>
Merge pull request #6691 from jamienoss/issue11522-support-memcached-b4-pool-removed

HPCC-11522 libmemcached plugin

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>