Fix DynamoDBLeaseTaker logging of available leases (#846)

When `DynamoDBLeaseTaker` is deciding what leases to take to satisfy the
target lease-count of it's worker, it can either take *expired* leases
that haven't been updated by any other worker for a while, or failing
that, 'steal' a limited number (by default, 1) of leases from other
workers, in order to ensure its worker is gradually getting closer to
it's target.

It turns out that the log message it prints out when taking expired
leases is misleading. For instance, in this message - the leases
weren't stolen, they were taken from the available 'expired' leases:

```
Worker xxxxxx saw 68 total leases, 0 available leases, 6 workers. Target is 12 leases, I have 2 leases, I will take 10 leases
```

...yet the message says there were 0 available leases, so how is that
possible? The truth is, there were 10 available expired leases, but the
log message was printing out the value of `expiredLeases.size()` - and
`expiredLeases` has been mutated over the course of the method, with
entries removed and added to `leasesToTake`:

0c5042dadf/amazon-kinesis-client/src/main/java/software/amazon/kinesis/leases/dynamodb/DynamoDBLeaseTaker.java (L421)

The fix to logging in this commit just records `numAvailableLeases` at
the start of the method, so we can give an accurate log message:

```
Worker xxxxxx saw 68 total leases, 10 available leases, 6 workers. Target is 12 leases, I have 2 leases, I will take 10 leases
```

amazon-kinesis-client/src/main/java/software/amazon/kinesis/leases/dynamodb/DynamoDBLeaseTaker.java

@@ -347,6 +347,7 @@ public class DynamoDBLeaseTaker implements LeaseTaker {
         MetricsUtil.addWorkerIdentifier(scope, workerIdentifier);
         List<Lease> veryOldLeases = new ArrayList<>();
 
+        final int numAvailableLeases = expiredLeases.size();
         int numLeases = 0;
         int numWorkers = 0;
         int numLeasesToReachTarget = 0;
@@ -438,7 +439,7 @@ public class DynamoDBLeaseTaker implements LeaseTaker {
                 log.info(
                         "Worker {} saw {} total leases, {} available leases, {} "
                                 + "workers. Target is {} leases, I have {} leases, I will take {} leases",
-                        workerIdentifier, numLeases, expiredLeases.size(), numWorkers, target, myCount,
+                        workerIdentifier, numLeases, numAvailableLeases, numWorkers, target, myCount,
                         leasesToTake.size());
             }