Logging for PostGIS plugin

[artemp]

Would be useful to think through a robust and flexible logging mechanism to quickly hone in on layers and query performance without being in compiled debug mode.

Would target the PostGIS plugin but potentially also the SQLite/oracle plugins.

Potentially could expose options like:

{{{
yes
/tmp/mapnik.log
}}}

or

{{{
stdout/stderr/file/etc
}}}

[kunitoki]

@artemp

Look at https://github.com/mapnik/mapnik/compare/debug-plugins there is some progress in this regard (both debug and statistics). It's a starting point and needs some refinements, but it's a start.

[kunitoki]

This was already implemented in #937. Closing.