Panic rather than fall into an unrecoverable state

Fix test

Run Rebalance test many times to pick up concurrency bugs

Don't handle interrupt signals

Let any program that uses this library handle them

Propagate session to dynamo and cloudwatch clients

Avoid race condition

Disable retries to prevent accidental duplicate entries

Configure dynamo throughput

Default to pay per request
allow configuring provisioned throughput

Use struct composition

Merge pull request #4 from patrobinson/race-condition-get-lease

Expose race conditions in GetLease