[SPARK-18796][SS] StreamingQueryManager should not block when starting a query

## What changes were proposed in this pull request?

Major change in this PR:
- Add `pendingQueryNames` and `pendingQueryIds` to track that are going to start but not yet put into `activeQueries` so that we don't need to hold a lock when starting a query.

Minor changes:
- Fix a potential NPE when the user sets `checkpointLocation` using SQLConf but doesn't specify a query name.
- Add missing docs in `StreamingQueryListener`

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16220 from zsxwing/SPARK-18796.