Async chain

[bdeggleston]

This PR relates to https://github.com/apache/cassandra-accord/pull/17

[dcapwell]

FYI need the following so tests can run

diff --git .build/include-accord.sh .build/include-accord.sh
index 0dc01bf2ba..079c96da1c 100755
--- .build/include-accord.sh
+++ .build/include-accord.sh
@@ -24,8 +24,8 @@ set -o nounset

 bin="$(cd "$(dirname "$0")" > /dev/null; pwd)"

-accord_repo='https://github.com/apache/cassandra-accord.git'
-accord_branch='trunk'
+accord_repo='https://github.com/bdeggleston/cassandra-accord.git'
+accord_branch='async-chain'
 accord_src="$bin/cassandra-accord"

 checkout() {