Partial bug #2964 fix: Prevent memory leaks when ICAP transactions fail.

Partial bug #2964 fix: Prevent memory leaks when ICAP transactions fail.

We now make sure that heap-allocated objects are deleted if an exception
is thrown before the object pointers are saved/registered in a safe location
like a data member.

Assigning state.serviceWaiting=true after calling callWhenReady() in ModXact
prevents ModXact leak when callWhenReady() throws. This may need more work
to mark ModXact state appropriately for the adaptation log.

TODO: Convert other HttpMsg pointer members to use safe HttpMsg::Pointer.