Allow user specified host in connection with sentinel

[koterevma]

Hi! It's my first time contributing to an open source project, so feel free to point out if i made something wrong

What is the problem?

I am trying to use redis as an external cache for latency-critical application that is scaled on three different physical servers

So for each application instance situation looks like this:

We have one redis instance that is available locally and two redis instances that are available but will cost a few net hops to reach (we want to avoid that at all costs). Preferred behavior is to always send readonly operations to local redis instance if possible, and if it is not alive, then we must go to any other available instance

The main issue is net latency with Redis client created with Sentinel and configured to read from Role::SLAVE because it chooses random host, but I want to specify preferred one

Suggestion

Allow user to specify host+port in ConnectionOptions and if it is suitable according to configuration, then use it

If user leaves host unspecified the behavior of connection pool with sentinel will be the same - randomly choose host from available replicas

Example

Before changes:

// Assume that we are running on server-1 and redis-instance-1 has the least network delay
connection_opts.host = "redis-instance-1";
connection_opts.port = 6379;
// While creating ConnectionPool we silently ignore host specified by user earlier
auto slave = Redis(sentinel, "master_name", Role::SLAVE, connection_opts, pool_opts);
slave.get("key");  // Possible net hops, random

After changes

// Assume that we are running on server-1 and redis-instance-1 has the least network delay 
connection_opts.host = "redis-instance-1";
connection_opts.port = 6379;
// Not ignoring host specified by user
auto slave = Redis(sentinel, "master_name", Role::SLAVE, connection_opts, pool_opts);
slave.get("key");  // Guaranteed low latency if redis-instance-1 is alive and is has role "slave"

// The same behavior as before changes: 
connection_opts.host = "";  // Set to empty string or leave unspecified
auto slave = Redis(sentinel, "master_name", Role::SLAVE, connection_opts, pool_opts);
slave.get("key");  // Connect to randomly selected replica