ENG-5298: Fix double deletion when both elastic context and COW context are in
use.

If a tuple is pending delete by the COW context, the elatic context shouldn't
try to delete it again. Let the COW context take care of it after it's safely
scanned.
Demote a log message in activating table stream.

The elastic index can be missing when a index read request comes in in normal
operation. Leave the decision of printing the error message to the caller.
Merge pull request #886 from VoltDB/ENG-5298

ENG-5298: Fix double deletion when both elastic context and COW context are in use