If you open multiple databases, a static reference to the WT_EXTENSION_API
can fail, if the second database to be opened closes before the first
database to be opened, because the close of the second database zero's out
the WT_EXTENSION_API values for the shared library, which the first database
then tries to use.

Get rid of all of the statics in all of the compressors.
Remove the WT_COMPRESSOR documentation from examples/c/ex_all.c (we're already
using ext/compressors/nop/nop_compress.c for documentation purposes and it's
better code), use it everywhere.
Add support for periodic hot backups to test/format.
The WT_COMPRESSOR field has to be a structure, not a pointer.
And fix the code that uses it.
Fail any attempt to perform a hot backup on a database where there
backing objects of a type other than file or LSM.
Don't attempt hot backups on non-standard data types, that's an error now.
Add WT_PREFIX_MATCH_LEN, a version of WT_PREFIX_MATCH that doesn't go
past the specified length of the string.
Check we can't name checkpoints that include LSM tables.
Change the complex_populate fuction to optionally create LSM tables,
change the backup test to backup LSM trees.
Disallow hot backups in any tree that includes something other than a
"file" or "lsm" type or source entry.

LSM objects don't have "source=XXX" entries, so we can't use that to
figure out what files we need.  Rewrite the table-level backup to use
the table name in all cases, instead.  This isn't always going to work,
though, we could match non-unique table names, that is, "table_X" and
"table_XX" will both match a backup target of "table_X".
Typo, test was flipped.
Populate with LSM for the named-checkpoint test.
Update backup to use __wt_schema_worker. Stop LSM drops during backups.
path variable no longer used, don't return without freeing the scratch buffer.
Merge branch 'develop' into hot-backup-schema
avoid spelling complaint
Delete test_illegal_objects, now that we're using schema_worker,
there's no special path for illegal objects and no reason to do
any special testing.
Re-work the code to build the target list, uncomment object checking,
this now works.
We have to write all of the names/value pairs out to the backup file,
not just the file: objects, don't discard the trailng ".wt" from the
file object name, whatever is in the metadata file is what we use.