[IMP] fields: use a "lazy" cursor to compute attribute `digits`

Borrowing a cursor each time you access `field.digits` may be costly, because
of the connection reset.  Moreover, in most cases, the cursor is not used at
all, since the decimal precision are kept in cache.

For the installation of module `product` with its demo data, the number of
cursor allocations was reduced from ~1500 to about 50!