Removing assert() calls from public headers

Removed calls to assert() from public headers to have
full control when assertions are enabled in wsrep-lib
code regardless of parent project build configuration.
Moved methods containing assertions and non-trivial
code from headers into compilation units.

dbsim/db_storage_engine.cpp

@@ -20,6 +20,8 @@
 #include "db_storage_engine.hpp"
 #include "db_client.hpp"
 
+#include <cassert>
+
 void db::storage_engine::transaction::start(db::client* cc)
 {
     wsrep::unique_lock<wsrep::mutex> lock(se_.mutex_);