IMAP to IMAPx migration is inefficient, it lefts the IMAP cache for the IMAPx, which leads to other issues, like console warnings and semi-constant folder redownloads, which is painful with large folders.

Steps:
a) create a backup of an IMAP account with something in its cache
b) restore from this backup (on git master)

- see the corresponding .source is imapx
- the corresponding ~/.cache/evolution/mail/... folder still contains cache
  of the original IMAP

The issue might be that the migration code moves old cache to trash, but the cache-reaper restores it. I verified it with gdb:

Adding 1352390386.11787.5@zyxPad ('Source_3')
Converting 1289392855.27592.1@zyxPad from IMAP to IMAPX

Breakpoint 3, evolution_source_registry_migrate_imap_to_imapx (server=0x78c6c0 [ESourceRegistryServer], key_file=0x880340, uid=0xba0fa0 "1289392855.27592.1@zyxPad")
    at evolution-source-registry-migrate-imap-to-imapx.c:75
75		g_key_file_set_string (key_file, group_name, "BackendName", "imapx");
(gdb) 
Continuing.

(evolution-source-registry:4370): evolution-source-registry-WARNING **: Failed to move '/home/mcrha/.cache/evolution/mail/1289392855.27592.1@zyxPad' to trash: No such file or directory

Breakpoint 1, cache_reaper_move_directory (source_directory=0xb5bc00, target_directory=0xa98220) at module-cache-reaper.c:210
210			g_file_move (
1: g_file_get_path (source_directory) = 0xb698f0 "/home/mcrha/.cache/evolution/mail/trash/1289392855.27592.1@zyxPad"