eCAP APIs for HostTransaction::{virgin, cause, adapted} incorrect constness

[Mark]

In `src/libecap/host/xaction.h`, these methods are defined as non-const and their return types are also non-const, but they should be. Callers cannot rely on the host to know of changes to the structures (e.g., if adapter edits the Body) through this API. Therefore, they should be changed to return const references.

Current:
virtual Message &virgin();
virtual const Message &cause();
virtual Message &adapted();

Proposed:
virtual const Message &virgin() const;
virtual const Message &cause() const;
virtual const Message &adapted() const;

Initial testing suggests that sample adapters, clamAV sample, and Squid compile successfully with the above changes. This should probably be verified.