[FEATURE] Index refresh job management phase 2

[dai-chen]

Is your feature request related to a problem?

In Flint 0.1, #57 added RECOVER INDEX JOB statement for index refresh job restart. There are other useful management statements still missing, such as show index refresh status and progress, stop index auto refresh etc.

What solution would you like?

1. SHOW INDEX JOB statement shows job info that may include:
   a. Job status (refreshing or terminated)
   b. Progress (how many files are refreshed)
   c. Last update time (heartbeat timestamp)
2. CANCEL INDEX JOB statement terminates the refresh job (index data will be stale later). Example use includes:
   a. Current index data is sufficient for user;
   b. Gracefully stop the refresh job before internal deployment

[dai-chen]

The requirements here is partially met in the following work:

1. [FEATURE] Show Flint indexes support #265: Show Flint index statement display both index metadata and state
2. [FEATURE] Support for ALTERING options of existing flint indices. #278: Alter index statement makes more sense than cancel, otherwise auto refresh index semantic is violated

Closing and reopen in future as needed.