Avoid linear search in StmtCache

[jonhoo]

The StmtCache currently does a linear probe of the order vector (including a string comparison for each position) every time it looks up a query. This is fine if the statement cache is fairly small, or a small set of queries are very common, but if it is larger, this scan + remove + push can become quite costly. From what I can tell, it is only used for keeping an LRU of the items in the HashMap. If the values in the map were instead, say, (usize, InnerStmt), where the usize is an incrementing number kept in the StmtCache, then the scan could happen only on eviction, rather than on every lookup.