Multi-value insert for MySQL backend

[Daniel Schwen]

Hello List,
when working with GnuCash 'Budgets' using a remote MySQL connection as a
storage backend long delays occur a GnuCash loops over tons of single value
INSERT operations. It would be useful to provide a method in
gnc-backend-sql.cpp to prepare and execute insert statements that build a
list of values (for example by passing in a double pointer
to GncSqlColumnTableEntry and a row number (there'd have to be some
validation that each row has the same column names though).
Daniel