Add sqlite_dbpage extension build tag

[hifi]

Provides raw access to database pages. Useful for taking consistent live snapshots of WAL databases without causing write locks by reading out all of the pages and writing them either to a file or some external storage.

The sqlite3_rsync utility that is bundled with sqlite3 source is one way to use it to somewhat efficiently synchronize a remote database.

Fixes #1288

[mattn]

Can you write a test?

[hifi]

Added a test that makes a full copy of a database using sqlite_dbpage table and checks the copy is consistent and has the expected data.