NEW ISSUE: Provide some way to prevent early binding to global domainchannels

Title:  Provide some way to prevent early binding to global domain channels

Target: Assembly 1.2 WD 03

Description:

As currently drafted, producers and consumers can indicate global domain 
channels in their @source & @target attributes.

In some scenarios, assemblers will want to prevent the use of said 
references to global domain channels.  For example, an 
assembler/deployer may wish to deploy their composite in conjunction 
with an application provided by a 3rd party. Since the third party is 
unlikely to know the details of the target SCA domain, they cannot 
safely reference global domain channels. To that end, the 
assembler/deployer wants to be sure that no such global domain channels 
are referenced by accident.

Currently, there's no specified way for the SCA runtime to enforce such 
a constraint.

Proposal:

(Broad strokes:) In section 5.11, add an attribute to the 
implementation.composite element called @allowGlobalRefs (defaults to 
true). When this is set to false, any contained use of @source or 
@target that refers to a global domain channel MUST trigger an error 
when contributed to the domain.