When processing long ranges of blocks, the account cache grows unbounded
which cause huge memory spikes.
Here, we move the cache to a second-level cache after each block - the
second-level cache is cleared on the next block after that which creates
a simple LRU effect.
There's a small performance cost of course, though overall the freed-up
memory can now be reassigned to the rocksdb row cache which not only
makes up for the loss but overall leads to a performance increase.
The bump to 2gb of rocksdb row cache here needs more testing but is
slightly less and loosely basedy on the savings from this PR and the
circular ref fix in #2408 - another way to phrase this is that it's
better to give rocksdb more breathing room than let the memory sit
unused until circular ref collection happens ;)
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