TACHYON-253 Add volatile modifier to EditLogProcessor.mIsStandby to guarantee visibility

The EditLogProcessor.mIsStandby is being read and modify by different threads.

There could be code optimization with memory barrier that could cache the variable changes among threads.
Add volatile modifier to let JVM guarantees that every read of the variable would from main memory instead of CPU cache.

And small typo fix from "Standy" to "Standby"