lib/db: Add db and test with invalid files (#4954)

This adds a couple of utilities for transporting databases in JSON and a
test to load a database and verify a couple of invalid bits. The test
itself is quite pointless at the moment, but it lays the groundwork for
testing the migration of this data in the next step (after the invalid
bit should be changed to local flags for local files).

When that happens we need to have a database in the old format already
there in order to be able to test the migration.

lib/db/testdata/v0.14.48-ignoredfiles.db.jsons

@@ -0,0 +1,15 @@
+{"k":"AAAAAAAAAAABYmFy","v":"CgNiYXJKBwoFCAEQ6QdQAg=="}
+{"k":"AAAAAAAAAAABZm9v","v":"CgNmb284AUoHCgUIARDoB1AB"}
+{"k":"AAAAAAAAAAACYmF6","v":"CgNiYXo4AUoHCgUIKhDoBw=="}
+{"k":"AAAAAAAAAAACcXV1eA==","v":"CgRxdXV4SgcKBQgqEOoH"}
+{"k":"AQAAAABiYXI=","v":"CisKBwoFCAEQ6QcSIP//////////////////////////////////////////"}
+{"k":"AQAAAABiYXo=","v":"Ci0KBwoFCCoQ6AcSICoAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAGAE="}
+{"k":"AQAAAABmb28=","v":"Ci0KBwoFCAEQ6AcSIP//////////////////////////////////////////GAE="}
+{"k":"AQAAAABxdXV4","v":"CisKBwoFCCoQ6gcSICoAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"}
+{"k":"BgAAAAAAAAAA","v":"dGVzdA=="}
+{"k":"BwAAAAAAAAAA","v":""}
+{"k":"BwAAAAEAAAAA","v":"//////////////////////////////////////////8="}
+{"k":"BwAAAAIAAAAA","v":"KgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA="}
+{"k":"CQAAAAA=","v":"CicIATACigEg//////////////////////////////////////////8KJwgCMAKKASD4+Pj4+Pj4+Pj4+Pj4+Pj4+Pj4+Pj4+Pj4+Pj4+Pj4+AolCAGKASAqAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABCoudnCuM+vlxU="}
+{"k":"CwAAAAAAAAAAAAAAAQ==","v":"AAAAAAAAAAABZm9v"}
+{"k":"CwAAAAAAAAAAAAAAAg==","v":"AAAAAAAAAAABYmFy"}