HPCC-12127 Use 64-bit integer for HTTP Content-Length header

The existing ESP code uses one of two member variables to set
HTTP Content-Length header: m_content_length64 (int64) and
m_content_length (int). When an HTTP message content is very
large, m_content_length may have an incorrect value. ESP
fails to download a >4G file due to the incorrect value. This
fix changes the m_content_length to be int64 and removes the
m_content_length64.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>