When ignoreLockingMode option is used, we default to the unix-none or win32-none
VFS, and enforce the connection to be read-only, because it's trivial to
corrupt data using a no-lock VFS.
Unfortunately read-only mode doesn't allow to open a WAL database if the journal
files are missing, unless the device is marked as "immutable".
There is no SQLITE_OPEN_* flag for immutable, and adding one could easily be
misinterpreted since it only makes sense for read-only.
This implement a readonly-immutable-nolock VFS shim on top of unix-none or
win32-none that overrides device characteristics with immutable. When
ignoreLockingMode option is specified, we use this VFS. It can only be used on
read-only connections, as enforced both by Storage service and a MOZ_ASSERT.
Differential Revision: https://phabricator.services.mozilla.com/D162492
