Merge prioritized configuration tables

You get several configuration tables, each a list of key=value strings with an integer priority. Merge them so that every key takes its value from the highest-priority table that sets it; between tables of equal priority, the later one wins. Then return key=value for a requested list of keys, in the order requested, skipping keys with no value.

Entries must be validated: a key is an identifier, and a value is a 32-bit signed integer written without leading zeros or a plus sign. A bad entry is ignored without affecting the rest of its table. Do it in one pass over the tables, without sorting every entry by priority.