[chainstate] store large MARF tries in a separate flat file

[jcnelson]

Once a blob gets to be bigger than the sqlite page size (which defaults to 4096), loading data from a blob starts to become expensive. Per sqlite's own benchmarks [1], once the blob size exceeds 100kb, reading from a file becomes faster no matter what page size is used. This suggests to me that the TrieFileStorage system should maintain a separate flat file for storing tries that exceed 100kb. This is most of them for bigger blocks -- as of right now, 31,428 out of 54,400 tries in the vm/clarity/marf.sqlite file (almost 60%) exceed 100kb. The sqlite db would instead store an offset and length in this file of the trie blob, and provide a Read + Seek-implementing struct for accessing it.

[1] https://www.sqlite.org/intern-v-extern-blob.html

[jcnelson]

In a test with 16384 MARF inserts over 32 blocks, the performance difference in reading nodes is over an order of magnitude better for tries stored in a flat file:

Total nodes read: 2,527,929
Total time spent reading nodes stored in external file: 7,667,938,853 ns
Total time spent reading nodes stored in SQLite blobs: 109,299,929,610 ns

It takes just over 3 microseconds to read a node if it's in an external file. It takes about 43 microseconds if it's in a SQLite blob.

Also, this is with opt-level = 3.