Add support for transaction update using /txn of the Consul API

[ecourreges-orange]

Hello,
I don't know how much work it represents yet, but would you be willing to accept my pull request if I added a config switch that makes all updates atomic using transactions with the /txn path of Consul?
In my case a restore of a couple thousand of keys takes more than 10s, and the configuration is mostly invalid during that interval, so my clients that reload instantly the changes will probably end up with an invalid configuration.

A transaction prevents that kind of issue, the client receives one notification of change at the end of the transaction.

Thank you.