ECPG: Increase the readahead window permanently if the application
ever sends us FETCH FORWARD/BACKWARD/RELATIVE with an argument
larger than the current readahead window.

This invalidates the points and corner cases tested by

    src/interfaces/ecpg/test/sql/cursor-ra-fetch.pgc

but not its results. Leave this test in place for reference.
ECPG: During FETCH ALL, temporarily use FETCHALL_MULTIPLIER times
the current readahead window size. FETCHALL_MULTIPLIER == 1000.
No change in regression tests.