Optimize HashList cache growth

[arnetheduck]

Currently, the cache in HashList is grown one element at a time - this leads to a lot of reallocation and re-hashing work when adding multiple items to the list, which is pretty common - common enough that I can see it coming up when measuring with a profiler.

There are lots of ways to grow lists with different tradeoffs in terms of memory use vs reallocation - based on gut feeling alone, I'd grow it 16 items at a time instead of 1 which likely would fix things for the practical use cases that we have.

[tersec]

Often, the ultimate size is known in advance (e.g., the number of validators), which would provide an optimal allocation strategy.

[arnetheduck]

actually, that might be the answer here: a lazy strategy for growing the cache - we don't need the cache until someone asks for a root that is affected by the new entry - queries for all old entries can be routed to the old cache while cache-growing can be delayed until a root affected by the new entries is needed