Use thread-local readers & inserters more pervasively

Now stored against ObjectDB rather than Repo, makes more sense as the
ObjectDB is the real creator of those readers & inserters, and it's
what they specifically act on.

Reduces object creation, memory consumption, increases speed:

$ time java -jar bfg-threadlocal.jar -D pushgems.rb rails.git

Cleaning
--------

Found 41613 commits
Cleaning commits:       100% (41613/41613)
Cleaning commits completed in 19,140 ms.

BFG run is complete!

real	0m23.714s
user	0m47.888s
sys	0m7.980s


$ time java -jar bfg-objectdb-threadlocal.jar -D pushgems.rb rails.git

Cleaning
--------

Found 41769 commits
Cleaning commits:       100% (41769/41769)
Cleaning commits completed in 14,064 ms.

BFG run is complete!

real	0m18.119s
user	0m35.120s
sys	0m6.204s