[WIP] [TACHYON-152] Client write refactor

Rewriting how client writes out blocks to make it easier to add remote blocks.

The current design is as follows:

Users requests access to a block

```java
Block block = Blocks.createBlock(tachyonFS, blockId, blockCapacityByte);
```

From there, the user can write to it by accessing its `Channel`.

```java
WritableBlockChannel channel = block.write();
channel.write(byteBuffer);
channel.close();
```

When the block is full, write returns -1, so you do this in a loop and create new blocks.

```java
while (buffer.remaining() > 0) {
  int n = mCurrentBlockChannel.write(buffer);
  if (n <= 0) {
    // block full, get new block
    getNextBlock();
  }
}
```
To make it work for remote write, what else does BlockHandler need to do?
BlockHandler could implement ByteChannel for better compatibility with other code.
I have tried locally, it works.