Fix memory leak due caused due to X509_STORE (yhirose#671)

* Fix memory leak due caused due to X509_STORE

* Add test for repro and address sanitizer to compiler flags

* Add comment

* Sync

* Associate ca_store with ssl context within set_ca_cert_store()

* Split SlowPost test

* Fix yhirose#674

Co-authored-by: yhirose <yuji.hirose.bug@gmail.com>

Author: omjego