added support to enabling database when ios is locked

[vzaidman]

if root.sqlitePlugin.enableDatabaseWhenLocked is not setted, plugin is using the default security.
if setted to true ios would permit an open before lock database to be used.
if setted to false ios would forbid any use of the file when it is locked.

[brodycj]

Thanks for the contribution and my apologies for the delay. Can you elaborate about when you would expect the sqlite database file to be locked inadvertently, i.e. "application forgets to close the database", "application crashes", etc. and also what the sqlite document says about the file locking?

If this is a common-place problem, I would consider enabling this fix by default. If this problem is caused by programmer error, I would prefer to document this problem very clearly, along with a possible workaround such as using the File API.

[vzaidman]

Ok, I'll explain briefly as I do not have the resources for a full research:

I encountered a problem when tried to write to the plugin's DB in a locked with a password iPad when in Background Fetch.

Looks like ios locks the file system when the device is locked and unless NSFileProtectionNone is set for a file (database file in our case) it cannot be accessed when the phone is locked. (code 0: "unable to open database file").

There are several possible values for the key NSFileProtectionKey of a file. None of them, but the specified above, let me write to the DB when iPad was locked.

The problem with this solution though, is that we might want to protect the database of our cordova application, and I believe NSFileProtectionNone shouldn't be the default value unless the user specifies he wants to give up on his database protection.

EDIT: Now, it looks like the whole documents directory should be set to NSFileProtectionNone

[brodycj]

Closing in favor of #786, thanks!