Update the Jenkins builds to build v4.2.x
TS-2554 New plugin to performance background fetch on Range responses
Added TS-2554 to CHANGES
TS-2672: allow mime_hdr_field_detach to be called on detached MIME headers

Allow mime_hdr_field_detach() to be called on detached MIME headers
so that TSMimeHdrFieldDestroy() can be called on a field that has
been detached by TSMimeHdrFieldRemove(). This matches the behavior
of mime_hdr_field_attach(), which silently ignores calls to attach
fields that are not detached.
TS-2672: add MIMEField::is_detached to match is_live

Add a matching is_detached accessor for the MIMEField readiness
state. Update all the non-assertion places that access
MIMEField::m_readiness directly.