Fix travis build errors for PostgreSQL

The errors were caused by executing queries with params in wrong order.

To fix the problem, this commit adds Database API functions to push the
current parameter number onto a stack and pop the value after query
execution, allowing build of multiple queries concurrently.

The caller can call db_param_push() when needed prior to starting the
build of a new query while another one is still pending execution.
db_query_bound() will call db_param_pop() after running the query.

Fixes 0014398