MINOR: http-ana: Add a txn flag to support soft/strict message rewrites

MINOR: http-ana: Add a txn flag to support soft/strict message rewrites

the HTTP_MSGF_SOFT_RW flag must now be set on the HTTP transaction to ignore
rewrite errors on a message, from HTTP rules. The mode is called the soft
rewrites. If thes flag is not set, strict rewrites are performed. In this mode,
if a rewrite error occurred, an internal error is reported.

For now, HTTP_MSGF_SOFT_RW is always set and there is no way to switch a
transaction in strict mode.