Add a test for blocking query on non-existent entry
Add a test for blocking query on non-existent entry

This test shows how blocking queries are not efficient when the query
returns no results.  The test fails with 100+ calls instead of the
expected 2.

This test is still a bit flaky because it depends on the timing of the
writes. It can sometimes return 3 calls.

A future commit should fix this and make blocking queries even more
optimal for not-found results.

hashicorp/consul,hashicorp/consul,hashicorp/consul,hashicorp/consul