Added seek before write in invalidate method. Without this, strange
things happen on NT and solaris. (I was probably breaking a rule of
thumb that says never to do a write() right after a read() even though
tell() would indicate I was in the right place.)

Added logic to update the cache index on an invalidate.

Added a separate call to check the cache size so that we only check
when a transaction is about to be written. It would be bad if we
"flipped" the cache twice during a transaction, as we would have
incomplete transaction data.  Note that this means that we could
exceed the target cache size if the data for a transaction exceeded
the target size (/2).