Change the apply trigger query cache to use a small footprint of the query

Change the apply trigger query cache to use a small footprint of the query
as the hash key. This key can be computed without doing any of the column
type lookups, making the query cache more efficient. If a prepared plan
is found, none of the catalog lookups are required at all.

Add a configuration option "apply_cache_size" to slon.conf.

Use custom wrappers around hash_any() and strcpm() to eliminate the use of md5.