Build Dynamo DB backed Message Store (#358)

* Work in progress...

* Finish first pass draft of MessagesDynamoDb

* Use begins_with everywhere for destination device id

* Remove now unused methods

* First basic test built

* Add another test case

* Remove comment

* Verify more of the message contents

* Ensure all methods are tested

* Integrate MessagesDynamoDb into the MessagesManager

This change plugs the MessagesDynamoDb class into the live serving
flow in MessagesManager.

Tests are not yet as comprehensive for this big a change as they
should be, but they now compile and pass so checkpointing here with a
commit.

* Put DynamoDB before RDBS when deleting specific messages

* Extract method

* Make aws sdk version into a property

* Rename clientBuilder

* Discard messages with no GUID

* Unify batching logic into one function

* Comment on the source of the value in this constant

* Inline method

* Variable name swizzle

* Add timers to all public methods

* Add missing return statements

* Reject messages that are too large with response code 413

* Add configuration to control dynamo DB timeouts

* Set server timestamp from the ReceiptSender

* Change to shorter key names to optimize IOPS

* Fix tests broken by changing column names

* Fix broken copyright template output

* Remove copyright template error text

* Add experiments to control use of dynamo and rds in message storage

* Specify instance profile credentials for the dynamic configuration manager

* Use property for aws sdk version

* Switch dynamo to instance profile credentials

* Add metrics to the batch write loop

* Use placeholders in logging

service/src/main/java/org/whispersystems/textsecuregcm/push/ReceiptSender.java

@@ -38,6 +38,7 @@ public class ReceiptSender {
 
     Account          destinationAccount = getDestinationAccount(destination);
     Envelope.Builder message            = Envelope.newBuilder()
+                                                  .setServerTimestamp(System.currentTimeMillis())
                                                   .setSource(source.getNumber())
                                                   .setSourceUuid(source.getUuid().toString())
                                                   .setSourceDevice((int) source.getAuthenticatedDevice().get().getId())