discovery: use a lower level but faster way to retrieve parents We already know that no revision in the undecided set are filtered, so we can skip multiple checks and directly access lower level data. In a private pathological case, this improves the timing from about 70 seconds to about 50 seconds. There are other actions to be taken to improve that case, however this gives an idea of the general overhead.

discovery: avoid computing identical sets of heads twice The very same set of heads is computed in the previous statement, it seems more efficient to just copy that result.

discovery: moved sampling functions inside discovery object In this patch, we transform the sampling functions into methods of the `partialdiscovery` class in the Python case. This will provide multiple benefit. For example we can keep some cache from one sampling to another. In addition this will help the Oxidation work as all graph traversal related logic will be contained in a single object.

discovery: rename `srvheads` to `knownsrvheads` The `srvheads` variable only contains the known set of remove heads. Renaming the variable make it clearer.

verify: small refactoring and documentation in `_verifymanifest` Small changes to make this area of code clearer.

verify: document the `_verifymanifest` method