Fix URI memoization errors

Why is this change being made?

This change fixes a few problems with memoization in the GitlabClient.

1. Change memoization attribute names to be the same as the method name.
2. Fix cases where different methods were using the same attribute for memoization, which could cause an incorrect URI to be returned.
3. merge_requests_search_uri should not be memoized, because the return value can change based on the positional argument.