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

In the existing ESP code, two member variables are used to store
and 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 replaces the
m_content_length64 with the m_content_length.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>