Generate a random salt when a page is encrypted before the salt is known - #633

A new database reads its salt from the first 16 bytes of the file and falls back to random bytes only when that read fails. The read happens when page 1 is first encrypted, after pager_write_pagelist has sent SQLITE_FCNTL_SIZE_HINT. A VFS that grows the file on that hint, like the unix VFS with SQLITE_FCNTL_CHUNK_SIZE or the browser opfs-sahpool VFS, makes the read return zeros, so every new database gets the same all-zero salt.

This generates the salt when a page is encrypted before the salt is known. A connection decrypts page 1 of an existing database before writing to it, so this only happens for a new file. Existing databases still read the salt from the header. beta has the same code.