Author: Christos Tsantilas <chtsanti@users.sourceforge.net>

Author: Christos Tsantilas <chtsanti@users.sourceforge.net>
Bug 2578: squid fails to resume dowload from FTP

The ftp code uses the FtpStateData::restart_offset and
FtpStateData::restarted_offset to compute the offset in the case of
partial responses, but it must also set the
ServerStateData::currentOffset member variable.
This patch:
  - completely removes the FtpStateData::restarted_offset member and uses
    the ServerStateData::currentOffset member variable instead.
  - adds the FtpStateData::setCurrentOffset(int64_t) and
    FtpStateData::getCurrentOffset() public methods to allow set/get the
    ServerStateData::currentOffset value