CREATE DATABASE ... STRATEGY WAL_LOG issues

Hi,

While hacking on my relation extension patch I found two issues with WAL_LOG:

1) RelationCopyStorageUsingBuffer() doesn't free the used strategies. This
   means we'll use #relations * ~10k memory

2) RelationCopyStorageUsingBuffer() gets the buffer for the target relation
   with RBM_NORMAL, therefore requiring a read of a block guaranteed to be
   zero

Easy enough to fix and shows clear improvement. One thing I wonder is if it's
worth moving the strategies up one level? Probaly not, but ...

Greetings,

Andres Freund