Slightly modified version of Herbert Toma's fix to reduce load time with

[David Hampton]

Log Message:
-----------
Slightly modified version of Herbert Toma's fix to reduce load time
with a large database of stock prices.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/backend/file:
        gnc-pricedb-xml-v2.c
    gnucash/src/engine:
        Period.c
        gnc-pricedb-p.h
        gnc-pricedb.c
        gnc-pricedb.h