Remove sqlite dependency in emscripten target, slow/large/unnecessary for web about netcraft HOT 5 CLOSED

Emscripten has IDBFS, IndexedDB based, https://kripken.github.io/emscripten-site/docs/api_reference/Filesystem-API.html

Found this when debugging, may not be representative of normal usage, but profiling often shows SQLite taking a lot of time:

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Maybe replace sqlite with a hand-tailored binary format? How many of the database-y features are really needed? Looks like it is fairly well abstracted in src/db.h and src/db.c (contains SQL).

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Would lmdb https://en.wikipedia.org/wiki/Lightning_Memory-Mapped_Database be a viable alternative? Or write something custom from scratch.

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There may be problems with the included sqlite compiling under emscripten, found this old pull request: emscripten-core/emscripten#1144 "Added SQLite3 distribution tailored for Emscripten ASM.js compilation" - it is said to remove function pointer casting for asm.js compilation. There is also https://github.com/kripken/sql.js (how does it differ from building sqlite with emscripten with no changes? is https://github.com/kripken/sql.js/commits/master/c/sqlite3.c unmodified?)

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Note there is a USE_CACHE option in src/config.h, set to 1. The database is always used in offline mode. But in online mode (used #localhost connecting to WebSandboxMC), changed to 0, comparing the performance graph in Chrome. Before (with USE_CACHE 1), see the sqlite3_exec calls:

after, with USE_CACHE 0, less heavy:

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