This adds an extra option to allow setting the SQLite page cache size as a regular option.
Prior to this commit it was only possible to set the SQLite page cache size via environment variables.
The option to use environment variables is preserved, and as options from the environment variable are applied after the new setting, environment variables take precedence.
The default value for the new option is to use 1% of the system memory for the page cache. For the restore process, a connection per worker may be made which defaults to half the number of cores, so the maximum amount of memory is 1% * half the CPU cores.
This is just the upper limit, and SQLite may choose not to use all of it.
This fixes#6178
Kenneth Skovhede
0a7a1cd689