Support sqlite builds with DQS=off

[knz]

If sqlite3 is built with double-quoted string literals disabled (which is recommended, see https://www.sqlite.org/compile.html#dqs), diskcache starts failing with the following crypting error:

no such column: "size"

This is because the syntax WHERE key = "size" in standard SQL refers to a column named "size", not a string literal. Parsing this as a string literal is a conditional sqlite extension.

This patch makes the code more robust by avoiding this optional sqlite feature.

[knz]

cc @grantjenks