Redis, async all the way through

[Beffyman]

This will avoid the potential thread block from the synchronous Connect & Ping.
Tools like Ben.BlockingDetector output the following:

warn: Ben.Diagnostics.BlockingMonitor[6]
     Blocking method has been invoked and blocked, this can lead to threadpool starvation.
        at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
        at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout)
        at StackExchange.Redis.ConnectionMultiplexer.ConnectImpl(Object configuration, TextWriter log)
        at StackExchange.Redis.ConnectionMultiplexer.Connect(String configuration, TextWriter log)
        at HealthChecks.Redis.RedisHealthCheck.CheckHealthAsync(HealthCheckContext context, CancellationToken cancellationToken)
        at Microsoft.Extensions.Diagnostics.HealthChecks.DefaultHealthCheckService.CheckHealthAsync(Func`2 predicate, CancellationToken cancellationToken)
        at System.Runtime.CompilerServices.AsyncMethodBuilderCore.Start[TStateMachine](TStateMachine& stateMachine)
        at System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1.Start[TStateMachine](TStateMachine& stateMachine)
        at Microsoft.Extensions.Diagnostics.HealthChecks.DefaultHealthCheckService.CheckHealthAsync(Func`2 predicate, CancellationToken cancellationToken)
        at Microsoft.Extensions.Diagnostics.HealthChecks.HealthCheckService.CheckHealthAsync(CancellationToken cancellationToken)
        at ...

[unaizorrilla]

Hi @Beffyman

Merged!! Thanks for contribute to this project!