Add document to explain High Availability support

High Availability (HA) support is important for large-scale and distributed systems like Tajo. As I know, Tajo at least supports HA for TajoMaster (TAJO-704). However, it is not clear how HA is supported for other components and how Tajo reacts in different situations. In the documentation, we should talk about it. For example, we can provide the answers for the following (or more) questions.

+ What happen if TajoMaster crashes ? for both cases,

• When there is no query running.
• When there is one (or more) query running

+ What happen if a TajoWorker crashes ? for both cases,

• When there is no query running.
• When there is one (or more) query running

For the above questions, the case when there is a running query is very important because we say "... Tajo is designed for both interactive and batch queries ... Tajo provides fault-tolerance ... for long-running queries ...".