Don't empty the method store when flushing the query cache

Don't empty the method store when flushing the query cache

When evp_method_store_flush() flushed the query cache, it also freed
all methods in the EVP method store, through an unfortunate call of
ossl_method_store_flush_cache() with an argument saying that all
methods should indeed be dropped.

To undo some of the confusion, ossl_method_store_flush_cache() is
renamed to ossl_method_store_cache_flush_all(), and limited to do
only that.  Some if the items in the internal ALGORITHM structure are
also renamed and commented to clarify what they are for.

Fixes #18150

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18151)