Add configuration parsing to the public API.

Don't remove it from the extension API for now - to match the value
packing code. Updated inline documentation, but not the official
documentation - since I suspect this will change before it's merged.
Update wtperf usage of configuration parsing API.
Add ability for wtperf to run with multiple databases in different threads.

This can be handy when testing shared cache behavior or generating different
I/O workloads.
Merge branch 'develop' into config-parse-api
Merge branch 'config-parse-api' into wtperf-multiple-databases
Update the configuration parsing API. Not finalized, but a step closer.
Update the configuration parsing API. Including:

* Don't copy the input string.
* Remove reset and remove complex semantics.
* Rename search back to get.
Add in an example for querying a nested value.
Configuration parser API, update documentation and examples.
Fix example code return value.
Update the configuration string parsing in Helium data source.
Fix compiler warnings re: ignored return values.
Add wiredtiger_config_parser_open and wiredtiger_version into extension API.

This is necessary so that statically linked applications can always
find the symbols.
Merge branch 'develop' into config-parse-api

Conflicts:
	src/include/wiredtiger_ext.h
Typo fix that got lost in merge.
Merge branch 'config-parse-api' into wtperf-multiple-databases

Conflicts:
	bench/wtperf/wtperf.c
	bench/wtperf/wtperf.h
Fix error handling in wtperf config parsing.
Update documentation for wiredtiger_config_parser_open
Add upgrade documentation about configuration string parsing changes.
Fixes to wtperf as per review comments.
Merge pull request #875 from wiredtiger/wtperf-multiple-databases

Add ability for wtperf to run with multiple databases in different threa...