Shared pool among clients and fixed pool resources release

[filipecosta90]

• [add] added option to create clients with shared pool, via NewClientFromPool(pool *redis.Pool, name string) *Client
• [fix] added option to releases the resources used by the pool ( single, and multi-host ) via Close()

fixes #16, #9

simple example of two clients sharing the pool:

        pool := &redis.Pool{Dial: func() (redis.Conn, error) {
        return redis.Dial("tcp", host, redis.DialPassword(password))
    }, MaxIdle: maxConns}
    client1 := NewClientFromPool(pool,"index1")
    client2 := NewClientFromPool(pool,"index2")

[codecov[bot]]

Codecov Report

Merging #51 into master will decrease coverage by 0.05%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   72.65%   72.59%   -0.06%     
==========================================
  Files          12       12              
  Lines        1064     1080      +16     
==========================================
+ Hits          773      784      +11     
- Misses        232      236       +4     
- Partials       59       60       +1     

Impacted Files	Coverage Δ
redisearch/pool.go	76.92% <66.66%> (-8.80%)	:arrow_down:
redisearch/client.go	79.75% <100.00%> (+0.31%)	:arrow_up:

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[filipecosta90]

@ericfengchao can you review this one as well? Would like your input but was not able to mark you as reviewer

[filipecosta90]

@itamarhaber and @ericfengchao can you double-check the changes :)

[filipecosta90]

Although the change to Close will only report the last error I'm good with that (is returning an array of errors something that's done in golang anyway?)

Now that you've mentioned it @itamarhaber I believe the best way is to keep the function signature but to generate an error with all details of error occurrences (I'll change it and return back in a comment to see if you guys like that idea ).

[filipecosta90]

lgtm 100 but ci was failing tho

It's failing only due to coverage difference -0.06% with the added error concatenation. I believe we can merge it :+1: