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

verify: document `_verifychangelog` We document the method input, output and checks.

verify: rename the `checklog` to `_checkrevlog` The method is for internal use only. In addition we make the method name explicitly contains `revlog` to make it clearer it is checking higher level revlog properties.

verify: document the `checklog` method Let us add details about what the function is expected to do.

revlog: add some documentation to the `checksize` method I had to look at it, so I figured I would leave some documentation for the next person seeking information.

verify: make `checkentry` a private method This method is for internal use only.

verify: document the `checkentry` method This method checks various core propertes of a revision. We document inputs, outputs and the checks performed.