pager: journal_mode=WAL switching

[iheitlager]

Scope

Switch between journal modes:

PRAGMA journal_mode=WAL;    -- journal → WAL
PRAGMA journal_mode=DELETE; -- WAL → journal

Implementation

Journal → WAL

1. Checkpoint any pending journal
2. Create WAL file + SHM
3. Set header flags

WAL → Journal

1. Checkpoint all WAL frames
2. Delete WAL + SHM files
3. Clear header flags

Phase

V6.3 (Concurrency)

Estimate

1 day

[iheitlager]

Closed via PR #426.