DERBY-6086: Factor out logic for creating custom connection in jdbcapi.StatementPoolingTest

Factored out logic for creating logical connections with statement caching into
a separate inner helper class. Used when you need to get more than one logical
connection from a single pooled connection.

Patch file: derby-6086-1b-logical_con_boilerplate.diff


git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1448956 13f79535-47bb-0310-9956-ffa450edef68