Give the possibility to control pool size of Mongo Data Connection

[TomaszGaweda]

By default it's min=zero, which is suboptimal for point-like queries (e.g. select by key). It takes more time to set up a connection than to perform actual query.

Checklist:

• Labels (Team:, Type:, Source:, Module:) and Milestone set
• Label Add to Release Notes or Not Release Notes content set
• Request reviewers if possible
• Send backports/forwardports if fix needs to be applied to past/future releases
• New public APIs have @Nonnull/@Nullable annotations
• New public APIs have @since tags in Javadoc

[frant-hartm]

There is no test.