Cleanup use of X509 STORE locks

Cosmetic changes to use the X509_STORE_lock/unlock functions.
Renamed some ctx variables to store.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9366)

(cherry picked from commit 7a9abccde7b7a5e36efe42d89246f6cfd4d59f44)