next_id should only query once inside the transaction

[atz]

Fixes #66

This preserves the API as it stands. Basically read and write! are for "outsiders" that only know about Noid::Minter. The serialize and deserialize methods are the meat.

If it is agreeable, I might go further to make write! look like:

def write!(minter, inst = instance)
  inst.update_attributes!(
    seq: minter.seq,
    counters: JSON.generate(minter.counters),
    rand: Marshal.dump(minter.instance_variable_get(:@rand))
  )
end

Then it can replace serialize completely.