fuzz log parser

[spacejam]

[divergentdave]

FYI, I tried hooking up cargo-fuzz to the read_message() function in src/pagecache/reader.rs, but it didn't turn up anything interesting. My WIP branch is at https://github.com/divergentdave/sled/tree/fuzz-logger-read-message. Instrumenting open_raw_log would give better coverage, but that would either require dumping fuzzer output to disk, and taking a big throughput hit, or some significant changes to Config and IoBufs to allow working from in-memory buffers only.