Options to use multiple brokers and presence managers

[FZambia]

A couple of options for Node's Config:

// GetBroker when set allows returning a custom Broker to use for a specific channel. If not set
// then the default Node's Broker is always used for all channels. Also, Node's default Broker is
// always used for control channels. It's the responsibility of an application to call Broker.Run
// method of all brokers except the default one (called automatically inside Node.Run). Also, a
// proper Broker shutdown is the responsibility of application because Node does not know about
// custom Broker instances. When GetBroker returns false as the second argument then Node will
// use the default Broker for the channel.
GetBroker func(channel string) (Broker, bool)
// GetPresenceManager when set allows returning a custom PresenceManager to use for a specific
// channel. If not set then the default Node's PresenceManager is always used for all channels.
// A proper PresenceManager shutdown is the responsibility of application because Node does not
// know about custom PresenceManager instances. When GetPresenceManager returns false as the second
// argument then Node will use the default PresenceManager for the channel.
GetPresenceManager func(channel string) (PresenceManager, bool)

This opens a way to interesting combinations – to inherit various channel options within one setup, opens new ways to scale.

Also, adding a possibility to have Publication.Channel which differs from subscription channel. This opens a road to utilize Broker's wildcard subscriptions. Wildcard subscriptions come with various limitations regarding Centrifuge features - such as join/leave events, presence, history, recovery - those won't work. But at the same time sometimes having a wildcard subscription with at most once delivery is necessary to keep the number of client channels small and thus achive very efficient consuming. I see how this may be especially useful with the addition of GetBroker option – because now it's possible to only use wildcards for a specific subset of channels. And still have all Centrifuge features for other channels.

[codecov]

Codecov Report

Attention: Patch coverage is 95.08197% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.74%. Comparing base (df55c9c) to head (87e0e91).

Files	Patch %	Lines
node.go	93.87%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #394      +/-   ##
==========================================
+ Coverage   81.60%   81.74%   +0.13%     
==========================================
  Files          40       40              
  Lines        7107     7127      +20     
==========================================
+ Hits         5800     5826      +26     
+ Misses        934      929       -5     
+ Partials      373      372       -1     

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