The grpc_polling_entity and grpc_httpcli_response types have default
member initializer values specified, to indicate what a cleared variable
of those types should have as values.
This file overrides that, however, by directly performing a memset to 0
over the structures. Instead, the initialization values defined by the
type should be honored.
Local types that include these types are also upgraded to specify
default member initializer values to simplify clearing them.
This fixes -Wclass-memaccess warnings in modern versions of GCC.