Log Message:
databases/mongodb3: Add, to ameliorate problems with mongodb 4

This is a copy of mongodb before the update to 4, with annotations to
explain why it exists.  Reasons include:

  - mongodb 4 is licensed under the Server Side Public License, to
    which some have objected

  - mongodb 4 requires c++17 (and hence gcc 8)

(ok to add during freeze by wiz@)