Existing querying was grouped together before handed over to the
datasource. This slowed down result display to however long the slowest
query took.

- create one query transaction per result viewer (graph, table, etc.)
  and query row
- track latencies for each transaction
- show results as soon as they are being received
- loading indicator on graph and query button to indicate that queries
  are still running and that results are incomplete
- properly discard transactions when removing or changing queries

Explore: reuse table merge from table panel

ux: remove duplicate placeholder attribute

[skip ci]

[skip ci]

* fix: text box template variable doesn't work properly without a default value

Add debug logging of dashboard permission checks

Stackdriver: Add possibility to authenticate using GCE metadata server