Redis Input Plugin using hiredis library

REQUIREMENTS:

* hiredis ( https://github.com/redis/hiredis.git )

USAGE:

This plugin has two current "modes" that it supports:

1. "queue"
The queue mode will LPOP or RPOP your message from a redis list.
Following parameters are required:
 - mode: Set mode to "queue" to enable the queue mode
 - key: The key to xPOP on
 - server: The name or IP address of the redis server
 - port: The redis listening port

Following parameters are optional:
 - password: If set, the plugin will issue an "AUTH" command before calling xPOP
 - uselpop: If set to "1", LPOP will be used instead of default RPOP

Redis pipelining is used inside the worker thread. The dequeue batch size is configured with the "batchsize" parameter (default is 10).

Imhiredis will query Redis every second to see if entries are in the list, if that's the case they will be dequeued
continuously by batches of "batchsize elements" until none remains.

Due to its balance between polling interval and pipelining and its use of lists, this mode is quite performant and reliable.
However, due to the 1 second polling frequency, one may consider using the `subscribe` mode instead if very low latency is required.

```
module(load="imhiredis")

input(
    type="imhiredis"
    mode="queue"
    key="vulture"
    server="127.0.0.1"
    port="6379"
    uselpop="1"
    password="foobar"
    batchsize="10"
)
```



2. "subscribe"
The subscribe mode will SUBSCRIBE to a redis channel. The "key"
parameter is required and will be used for the subscribe channel.

Following parameters are required:
 - mode: Set mode to "subscribe" to enable the subscribe mode
 - key: The key to subscribe to (aka the "channel")
 - server: The name or IP address of the redis server
 - port: The redis listening port

Following parameters are optional:
 - password: If set, the plugin will issue an "AUTH" command before listening to a channel
 - uselpop: If set to "1", LPOP will be used instead of default RPOP


```
module(load="imhiredis")

input(
    type="imhiredis"
    mode="subscribe"
    key="vulture"
    server="127.0.0.1"
    port="6379"
    password="foobar"
    batchsize="10"
)
```


TODO
* TLS support


