HPCC-8760 Upgrade ESP to upload multiple files per http request

Existing ESP can only upload one file per HTTP request. Now, the
function is revised to support multiple file upload per request.

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>
HPCC-8760 Revise based on review

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>
HPCC-8760 Revise one more time

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>
Merge pull request #3987 from wangkx/h8760

HPCC-8760 Upgrade ESP to upload multiple files per http request

Reviewed-By: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>