Read-only mode #404

To complement HarperFast/rocksdb-js#490, add read-only mode support for Harper. Obviously this means opening RocksDB in read-only mode (LMDB actually has a read-only mode as well). From my quick tests, the most obvious additional thing is to disable analytics (analytics/write.ts has a nice big switch for this, I think). I'd guess we probably want to control this with an env variable, but other possibilities include checking for having a harper parent process, or trying write mode with fallback to read-only mode. The known "use case" is for when NextJS launches build processes (and we aren't directly making the spawn call, it is somewhere in the NextJS code).