http.client: Remove use of PycURL's “reset” function

We don't re-use cURL objects anymore, so there's no need to reset them.
PycURL 7.19.0 has a reference counting bug leading to a crash after
a certain number of performed requests.

Since a unittest depended on this function for a test, it is replaced
with a per-request completion callback, which could also be used by
users of the HTTP client.

Unittests are updated and a test for the request object's “nice name”,
used by the lock monitor, is included.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>