Remove references to internal APIs

[achrinza]

MongoDB native driver v4.5.0 has marked existing APIs, of which Topology is utilised by this connector, as internal (PR: mongodb/node-mongodb-native#2810, issue: https://jira.mongodb.org/browse/NODE-3245). It should be noted that these APIs have always intended to be internal, and that the added internal markings in v4.5.0 are a formality.

This connector references Topology APIs at these locations:

• loopback-connector-mongodb/lib/mongodb.js

  Line 527 in 21a25fe

  !self.db.topology || (self.db.topology && !self.db.topology.isDestroyed())

• loopback-connector-mongodb/lib/mongodb.js

  Line 531 in 21a25fe

  } else if (self.db && !self.db.topology) {

This issue is to track replacing references to these APIs.