dht/KBucket: don't accumulate duplicate replacement entries

KBucket::pingQuestionable(const KBucketEntry &) stashes its argument (the prospective replacement KBucketEntry) in the KBucket::pending_entries QList if the number of outstanding pings of questionable entries is 2 or more. Unfortunately, the function was not checking for duplicate entries in the pending_entries list, which was causing unbounded memory usage as duplicate replacement entries would rapidly accumulate in the list.

This commit modifies the pingQuestionable function so that, before inserting a replacement entry into the pending_entries list, it first removes any matching entry from that list. Then, it prepends the new entry to the list (rather than appending as it did before this commit) since we want to consider the freshest replacements first when endeavoring to replace bad entries. To bound memory usage, the list of pending replacements is constrained to contain no more than K entries.