[Cdash] Allow multiple coverage submissions for the same buildid

[Nils Gladitz]

I am trying to submit both gcov and java emma coverage from the same 
build which I think leads to:

SQL error: ERROR:  duplicate key value violates unique constraint 
"coveragesummary_pkey"

Would it be possible to add a column to the coveragesummary table for 
the coverage type (e.g. gcov, emma) and make it part of the primary key 
so that multiple coverage submissions of different type become valid?

Would any other changes be required to make this work?

Nils