Reduce maximum coinscache size during verification

Due to growing coinsviewcaches, the memory usage with checklevel=3
(and standard settings for dbcache) could be up to 500MiB on a
64-bit system. This is about twice the peak during reindexing,
unnecessarily extending bitcoind's memory envelope.

This commit reduces the maximum total size of the caches used during
verification to just nCoinCacheSize, which should be the limit.