add advanced merge strategies

[WolframHempel]

Add merge strategies that will be executed when the client encounters a version exists error. The following strategies will be supported

C.MERGE_IF_NO_CONFLICT (default) current and incoming version will be merged if neither a value nor a tree conflict exists.

C.LOCAL_WINS the clients current version will be used

C.REMOTE_WINS the incoming update will be used

function(){} - a custom function with the following signature

function( localValue, remoteValue, recordName, recordVersion, callback ) {
    callback( error, mergedVersion );
}

merge strategies can be set in two places:
global as part of the client options

var ds = deepstream( 'localhost:6020', {
    /**
   * @ function mergeStrategy
   */
    mergeStrategy: C.REMOTE_WINS
});

per record on a per record basis

var rec = ds.record.getRecord( 'some/record' );
rec.setMergeStrategy( function(){...});

[WolframHempel]

I'm really in two minds whether this should be done on the client or the server.

Benefits of merging on the client
Greater control, less load on the server, possibility for user interaction / user merging

Benefits of merging on the server
Clients in different languages don't all have to implement merging, centralized, both old and new data of the record is already present, quicker, possibility of database / cache interactions

Love to hear some thoughts...

[anilgulecha]

@WolframHempel for the sake of this issue can you comment on what was chosen, and point to docs if any?