Revert "Add idle timeout to the connection pool (JAVA-419)."

This reverts commits f84cdc1c1b2bfa902528c0c1dc9bbf3cb75c3695,
38edc2e79982e00cc2727b00787054b9a297cc0f and
f5107501c2618acbd17a5ba38b0b310ccddcd7f4.

The implementation needs to be reconsidered (see JAVA-573). Due
to planning constraints, we are not able to address this in 2.0.9. The
feature will be reintroduced in a later version.