Add adapters/libhv

[ithewei]

Build steps:

git clone https://github.com/ithewei/libhv
cd libhv
./configure && make && sudo make install

git clone -b libhv https://github.com/ithewei/hiredis
cd hiredis
mkdir build
cd build
cmake .. -DENABLE_EXAMPLES=ON
make
./examples/example-libhv

output:

Connected...
argv[end-1]: ./examples/example-libhv
Disconnected...

[michael-grunder]

This PR looks good to me, but I can't find a great deal of information about libhv on the web, other than it has almost 5000 stars on GitHub.

@bjosv Any thoughts? I don't really see a downside to adding another adapter. If we merge it I'll update the Makefile too so it can build the example as well.

[bjosv]

I have tested the example and it works fine to build and run, so LGTM!

[bjosv]

The adapter is not supporting timeouts via ac->ev.scheduleTimer, but I guess that's ok.
It's only supported by 4 of the 9 current adapters.