add configurable maxIdleConns and maxIdleTime for sql databases

[arner]

Allows for use case specific tuning of database parameters.

    maxOpenConns: 20  # optional: max open read connections to the database. Defaults to unlimited. See https://go.dev/doc/database/manage-connections.
    maxIdleConns: 20  # optional: max idle read connections to the database. Defaults to 2.
    maxIdleTime: 30s  # optional: max duration a connection can be idle before it is closed. Defaults to 1 minute.

It also exposes a function Stats(): any to the database driver. This can be used for debugging or observability in general. In the case of SQL databases, this provides details about the connection pool (see go docs. For Badger, it returns the metrics for the underlying block cache.

[adecaro]

@arner , please:

• Can you add a comment to the description of the PR about the Stat function?
• Can you open a PR in the token-sdk that tests this commit?
  Many thanks :)

[arner]

Sibling PR at token sdk: hyperledger-labs/fabric-token-sdk#805

[adecaro]

LGTM