Add support for streaming pack / unpack to the API.

--HG--
rename : src/packing/packing_api.c => src/packing/pack_api.c
rename : src/packing/packing.c => src/packing/pack_impl.c
rename : src/packing/packing.c => src/packing/pack_stream.c
Move edits from (local) wiredtiger.h to wiredtiger.in.
Edit wiredtiger.in, not wiredtiger.h.
Add documentation and checking to pack streams, return the number of bytes (un)packed.
Fix pack_close: read fields before freeing the struct.
Add a schema generation number to avoid using stale table structures.

This is a stop-gap at best, until tables use the same caching and locking as btree handles.
typo
Add new exported symbols
Style cleanups: long lines, new spelling.
Commit a test to ensure session table cache is invalidated.
Merge pull request #461 from wiredtiger/pack-streams

Add support for streaming pack / unpack to the API.