Provide Constructor for MongoDB Connection Reusing

[ihsanciftci]

sharedb-mongo provides two ways of accessing a mongodb database:
a) takes a url, creates a client, connects to the server, accesses the database
b) takes a client, connects to the server, accesses the database

The second approach provides better flexibility and saves resource.

In fact, there is a better way:
c) takes a database

The third approach enables creating thousands of sharedb instances without hitting connection limitations since each connect() call creates another connection pool.

I'm not good at NodeJS but I changed the source code and it enabled running 4000 sharedb instances with 150 mongodb connections. I think this is a good result.

if (isLegacyMongoClient(client)) {
        self.mongo = self._mongoClient = client;
    }
    else if(client.s) {
        self.mongo = client;
        self._mongoClient = client.s.client;
        self._dbInstance = true;
    }
    else {
        self.mongo = client.db();
        self._mongoClient = client;
    }

ShareDbMongo.prototype.close = function(callback) {
  if (!callback) {
    callback = function(err) {
      if (err) throw err;
    };
  }
  var self = this;
  this.getDbs(function(err) {
    // Ignore "already closed"
    if (err && err.code === 5101) return callback();
    if (err) return callback(err);
    self.closed = true;
    if(!self._dbInstance) {
      self._mongoClient.close(function(err) {
        if (err) return callback(err);
        if (!self._mongoPollClient) return callback();
        self._mongoPollClient.close(callback);
      });
    }

  });
};

        const db = require('sharedb-mongo')({
            mongo: function (callback) {
                mongoPromise.then(client => {
                    let mongoDb = client.db(appId);
                    try {
                        callback(null, mongoDb);
                    }
                    catch (error) {
                        console.log(error);
                    }
                });

            }
        });

Notes:

Mongodb documents about connection pooling:

https://mongodb.github.io/node-mongodb-native/driver-articles/mongoclient.html#mongoclient-connection-pooling

"To reduce the number of connection pools created by your application, we recommend calling MongoClient.connect once and reusing the database variable returned by the callback:"

See #56 (comment) for another discussion about this feature request.