Fixes a bug that causes pg_cron to run during pg_upgrade

Fixes a bug that causes pg_cron to show up incorrectly in pg_stat_activity in PG11

- PostgreSQL 10 support

- Restrict the maximum number of concurrent tasks
- Ensure table locks on cron.job are kept after schedule/unschedule

* Fixes a memory leak that occurs when a connection fails immediately

* Fixes a memory leak due to switching memory context when loading metadata
* Fixes a segmentation fault that can occur when using an error message after PQclear

Initial 1.0 candidate

Update .pgpass comment in readme.

Fixes citusdata#5