This worklog implements support for stacked diagnostic areas and the
GET STACKED DIAGNOSTICS statement.

Each connection has it's own stack of diagnostic areas. The diagnostic area on
top of the stack is called the first diagnostic area. The diagnostic area just
below is the second diagnostic area. There can be more than just two diagnostic
areas, but only the top two are accessible.

Normally all interaction happens with the first diagnostic area. This is where
conditions (errors, warnings, notes) that are raised, are entered. The contents
of the first diagnostic area can be queried by the existing GET [CURRENT]
DIAGNOSTICS statement, while the contents of the second diagnostic area can be
queried by the new GET STACKED DIAGNOSTICS statement.

A new diagnostic area is pushed to the diagnostic area stack when a stored
routine handler is activated. Initially, this new area contains a copy of the
contents of the second diagnostic area (i.e. the first diagnostic area before
the push). This allows GET STACKED DIAGNOSTICS to be used throughout the handler
to inspect the conditions that triggered the handler, regardless of what happens
with the first diagnostic area.

This worklog also fixes
Bug#14342913 SP HANDLER DOES NOT PRESERVE CURRENT DIAGNOSTICS AREA 
If a stored routine handler exits with RESIGNAL, the first diagnostic area now
contains all conditions present when the handler was activated + the condition
added by RESIGNAL. Before this fix, only the first condition present at handler
activation was preserved.

The worklog also contains serveral refactorings of the error handling API.