Implementation of Dependency.release() and Depedency.reopen()

[superhellth]

Motivation:
Needed for the planned TUI plugin. Ensure TUI can stay open in one terminal, while doit jobs run in another. Generic interface provided for plugins to use.

Provided interface [new]:

• has_changes() -> bool: True if the backend holds changes not yet written to the file. It is always False once the DB is closed.
• release(discard=False): closes the handle without saving. It raises UnsavedChangesError if has_changes() is true and discard is not set. When it raises, the DB stays open and close() still saves.
• reopen(discard=False): calls release(discard), then reopens the file.

Main Cost:
To make that check reliable, all three backends (JsonDB, DbmDB, SqliteDB) now buffer every mutation in memory and touch the file only in dump(). A read-only user of the DB never writes to it or takes a write lock.

Changes:

• has_changes() on every backend, backed by a sticky _changed flag. set() and remove_all() set it. remove() sets it only if the id is in the file or in the dirty set. Reads never set it.
• UnsavedChangesError subclasses DatabaseException, so existing except DatabaseException handlers keep working. When it raises, the DB stays open and close() still saves.
• Deferred removals in DbmDB and SqliteDB. remove() and remove_all() record _removed / _truncate and apply them in dump(). Before, they deleted from the file immediately, so release() could
  not undo them. DbmDB.dump() tolerates keys another process already removed.
• SqliteDB.get() fix. It used to cache a missing row, so in_() then reported an id as present just because it had been read. It now caches only real rows, and in_() checks the file with a
  read-only query. This also fixes a misleading get_value() error for unknown task ids.
• Plugin backends without has_changes() or release() still work. Dependency uses getattr and treats a missing method as "no changes" or "keep handle".

Testing:

• New backend-parametrized suites cover has_changes(), the release guard, deferred removals, in_() after a read, and a sqlite3 no-write-lock check. They run on json, sqlite3, dbm.gnu,
  dbm.ndbm and dbm.dumb.
• test_release_does_not_save now uses release(discard=True).
• A get_status() regression test covers the checker_changed path. It fails if release(discard=True) is swapped for close().