(PUP-744) Only cache HTTP and verified HTTPS connections

Previously, it was possible for puppet to create an HTTPS connection
with VERIFY_NONE, and cache the connection. As a result, the pool code
could hand out an insecure connection, when the caller expected a secure
one (VERIFY_PEER).

In practice this isn't an issue, because puppet only uses insecure HTTPS
connections when bootstrapping its SSL key and certs, and that happens
before Configurer#run overrides the default non-caching pool
implementation. However, it could be an issue if a provider were to
request a insecure connection *during* catalog application, as that
connection would be cached, and possibly reused by puppet, e.g. when
sending the report.

This commit modifies the connection pool to only cache HTTP or secure
HTTPS connections. All other connections are closed after one HTTP
request/response.