XDCR explicit mapping allows multiple source collection to be mapped to a single target collection

Seems that rules like the following set is valid:

s1.c1 -> s1t.c1t

s1.c2 -> s1t.c1t

When we should really enforce a 1-to-1 mapping (i.e. no N-to-1 nor 1-to-N is allowed)

Currently, 1-N rule is not valid per mapping rules.