fix hex_random for v0.14

[sonots]

For fluent/fluentd#994
NameError: uninitialized constant Fluent::FileBufferChunk

Unifying logic of generating hex_random

Logic of generating hex_random was trivially changed. Now, the logic to generate hex_random was unified to get from chunk.unique_id. Before, it was generated from chunk.unique_id for FileBufferChunk and was generated from SecureRandom.hex for others.

Below is the list of how unique_id is generated. I analyzed that hex_random can be generated as unique_hex(chunk).reverse[0...@hex_random_length] for all of them with good randomness. So, I made codes common for all of them.

v0.12

FileBufferChunk

      now = Time.now.utc
      timestamp = ((now.to_i * 1000 * 1000 + now.usec) << 12 | rand(0xfff))
      tsuffix = timestamp.to_s(16)
      # why *2 ? frsyuki said that I forgot why completely.
      tsuffix.scan(/../).map {|x| x.to_i(16) }.pack('C*') * 2

MemoryBufferChunk

      now = Time.now.utc
      u1 = ((now.to_i * 1000 * 1000 + now.usec) << 12 | rand(0xfff))
      [u1 >> 32, u1 & 0xffffffff, rand(0xffffffff), rand(0xffffffff)].pack('NNNN')

LighteningBufferChunk

same with MemoryBufferChunk (as it extends MemoryBufferChunk)

v0.14

      now = Time.now.utc
      u1 = ((now.to_i * 1000 * 1000 + now.usec) << 12 | rand(0xfff))
      [u1 >> 32, u1 & 0xffffffff, rand(0xffffffff), rand(0xffffffff)].pack('NNNN')

[sonots]

tests failed, but seems not related with this commit.