VMOD using the synchronous hiredis library API to access Redis servers from VCL.

Highlights: * Full support for execution of Lua scripts (i.e. EVAL command), including optimistic automatic execution of EVALSHA commands. * All Redis reply data types are supported, including partial support to access to components of simple (i.e. not nested) array replies. * Redis pipelines are not (and won't be) supported. Lua scripting, which is fully supported by the VMOD, it's a much more flexible alternative to pipelines for atomic execution and minimizing latency. Pipelines are hard to use and error prone, specially when using the WATCH command. * Support for classic Redis deployments using multiple replicated Redis servers and for clustered deployments based on Redis Cluster. * Support for multiple databases and multiple Redis connections, local to each Varnish worker thread, or shared using one or more pools. * Support for smart command execution, selecting the destination server according with the preferred role (i.e. master or slave) and with distance and healthiness metrics collected during execution. * Support for Redis Sentinel, allowing automatic discovery of sick / healthy servers and changes in their roles.