Subject: [DISCUSS] Drop the support of jdk7 at a future 1.x release

The main reason is the asynchronous api we want to introduce in HBase
today. See HBASE-13784 and HBASE-16505.

The CompletableFuture in java8 is very suitable to use as the return value
of a async method. We can not use it if we still want to support java7, and
sadly, there is no candidate which is good enough to replace
CompletableFuture. ListenableFuture in guava or Promise in netty are good,
but we do not want to expose third-party classes in our public
API(especially guava, you know...). And we can also implement our own
ListenableFuture but it just a copy of guava. Or introduce a simple
Callback interface which does not need much code(for us) but this is a code
style around 2000s so people will not like it...

And luckily, I found that in our documentation

We only say that 1.3 will be compatible with jdk7, not all 1.x.

So here I propose that we drop the support of jdk7 in a future 1.x release,
maybe 1.4? Thus we can use CompletableFuture in both master and branch-1.