Bug in copySession method

What version of MongoDB are you using (`mongod --version`)?

What version of Go are you using (`go version`)?

1.12

What operating system and processor architecture are you using (`go env`)?

ubuntu, linux, amd

What did you do?

Trying to copy the session and have different socket timeout value. Sadly, if I set SetSocketTimeout on one session, value is changed on all sessions that were copied using copySession https://github.com/globalsign/mgo/blob/master/session.go#L844

This is because method creates new object of Session but reuses pointer values in object properties, e.g.: Session struct has property dialInfo *DialInfo. The new Session object takes pointer value from Session that should be copied :

scopy := Session{
    ...
    dialInfo: session.dialInfo,
}

SetSocketTimeout method sets values and changes are applied to all the sessions that were copied.

s.dialInfo.Timeout = d
s.dialInfo.ReadTimeout = d
s.dialInfo.WriteTimeout = d

There's more pointers in Session object that are "copied".
There's more methods that could have unexpected behavior.

Can you reproduce the issue on the latest `development` branch?

Probably, yes. Didn't see any related issues or PRs.

globalsign / mgo