Add support for header caching. It is currently unstable and should not
be used. It can be turned on by specifying "X_header_caching" to true in
the IMAP account's configuration.
The header cache is saved in a sqlite3 database in your XDG_DATA_DIR,
for example:
/home/epilys/.local/share/meli/17328072387188469646_header_cache.db
Concerns #31https://git.meli.delivery/meli/meli/issues/31
Manos Pitsidianakis