Remove dynamic ets tables from the httpc pool

This is a pretty straightforward swap to using the lists:key* BIFs
instead of an ets table. The number of HTTP clients is not expected to
ever exceed roughly 100 so this keeps the table size inside the
acceptable limits for a list.
Merge pull request #16 from cloudant/17425-remove-pool-ets-table

Remove dynamic ets tables from the httpc pool

BugzID: 17425