feat(offline/export): drop temp table after export done

[lianxmfor]

drop temp table after export is done.

TODO:

1. Unify offline/export and offline/join on drop temp table functions to offline.DbOpt

fix #1193

[wfxr]

lgtm

[lianxmfor]

In sync, the offline store is responsible for reading data, the online store is responsible for writing data, and the metadata store is responsible for modifying metadata. If all three stores are SQLite and use the same db file, offline delete the table and cause multiple connections to write errors: database is locked (5) (SQLITE_BUSY).

So it is better for offline to provide an interface: dropTempTable to be called by oomstore at the right time.

[lianxmfor]

This PR can't solve the problem of multiple writes for three stores under the same SQLITE file, close.