Schema grants of few principals are merged

[arshvin]

This line of code has a bug: if a catalog schema_grants map contains few principals with required grants, then these grants will be merged and applied for all of mentioned principals.
Let's imagine we have following:

...
    schema_grants = {
      "some-user-group1"                             = ["USE_SCHEMA", "EXECUTE", "REFRESH"],
      "some-user-name1"                              = ["SELECT", "APPLY_TAG"],
      "11111111-1111-1111-1111-111111111111" = ["ALL_PRIVILEGES"],
    }
...

Finally, all of these users will have following list of grants to mentioned schemas:

"some-user-group1"                             => ["USE_SCHEMA", "EXECUTE", "REFRESH","SELECT", "APPLY_TAG", "ALL_PRIVILEGES"]
"some-user-name1"                              => ["USE_SCHEMA", "EXECUTE", "REFRESH","SELECT", "APPLY_TAG", "ALL_PRIVILEGES"]
"11111111-1111-1111-1111-111111111111" => ["USE_SCHEMA", "EXECUTE", "REFRESH","SELECT", "APPLY_TAG", "ALL_PRIVILEGES"]