implement .insert on LCA_SqliteDatabase and/or support on-disk/lowmem lca index

[ctb]

In #1808, we are adding LCA_SqliteDatabase, which supports on-disk LCA databases.

The "standard" way of creating these is to use sourmash lca index ... -F sql, which has the disadvantage of first creating an LCA_Database in memory, and only then saving it to disk. This is unnecessarily memory intensive.

An alternative would be to directly support creation and update of an LCA database on disk in lca index, perhaps through the implementation of an insert on LCA_SqliteDatabase. This would be much lower memory.

Right now, you can do this by combining sourmash sig cat ... -o lca.sqldb (to create a SqliteIndex) then sourmash tax prepare -t <tax> -o lca.sqldb (to add a LineageDB_Sqlite table), but this skips all of the
consistency checking that sourmash lca index does.