EXTRA DURABLE build setting to improve robustness against corruption

[brody4hire]

As discussed in http://sqlite.1065341.n5.nabble.com/Is-WAL-mode-more-robust-against-corruption-td99624.html:

From https://www.sqlite.org/pragma.html#pragma_synchronous the default PRAGMA synchronous setting is FULL, wonder if it should be EXTRA instead.

Also waiting for more feedback if anyone else on the sqlite-users mailing list can find any possible cause for database corruption in this project. A couple possible causes for corruption on iOS that were recently fixed:

• Database malformation #703
• iOS threading issue with openDatabase? #716

UPDATE: it is desired to use compile-time option to enable this setting by default, details in comment below

ALSO WANTED: use recent SQLite version (3.22.0) to eliminate possibility of corruption due to outdated SQLite version

[brody4hire]

From https://www.sqlite.org/pragma.html#pragma_synchronous the default PRAGMA synchronous setting is FULL, wonder if it should be EXTRA instead.

Probably better to use SQLITE_DEFAULT_SYNCHRONOUS=3 setting ref:

• https://www.sqlite.org/compile.html#extra_durable
• https://www.sqlite.org/compile.html#default_synchronous
• https://www.sqlite.org/pragma.html#pragma_synchronous

NOTE that it looks like SQLITE_EXTRA_DURABLE is no longer supported ref: https://www.sqlite.org/compile.html#extra_durable (TBD information in https://www.sqlite.org/pragma.html#pragma_synchronous seems to be in conflict)

ADDITIONAL EXPLANATION: As I said after the response I got to http://sqlite.1065341.n5.nabble.com/Is-WAL-mode-more-robust-against-corruption-td99624.html I would expect that a mobile app may be terminated at any time and would not expect advanced notification to be 100% reliable, especially in a hybrid mobile stack, and should be robust against crashes including possible OS crashes due to bugs, battery issues, etc. This is why I am thinking it would be safest to use the compile-time equivalent to PRAGMA synchronous=3 (EXTRA).