[Java] Is RootAllocator thread safe ?

### Describe the usage question you have. Please include as many useful details as possible.

"Applications should generally create one RootAllocator at the start of the program, and use it through the BufferAllocator interface." refered from [memory management](https://arrow.apache.org/docs/java/memory.html)

It's thread safe and supposed to be defined as a "singleton" instance ?

### Component(s)

Java