Merge pull request #88 from manuzhang/at_least_once

add checkpointing
fix PriorityScheduler bug
Merge pull request #90 from manuzhang/at_least_once

fix OffsetManager update and checkpoint bug
add specs2 and offsetmanager test
Merge branch 'at_least_once' into add_test
change specs2 version
Merge pull request #91 from manuzhang/at_least_once

fix creating checkpoint topic conflicts
add offsetManagerSpec
Merge remote-tracking branch 'remotes/origin/at_least_once' into add_test
Merge pull request #92 from manuzhang/at_least_once

make consumer run in a separate thread from main loop
Merge remote-tracking branch 'upstream/master'
reallocate resource if executor launch rejected
Merge pull request #93 from huafengw/error_recovery

Error recovery
Merge remote branch 'upstream/master' into add_test
Merge remote branch 'origin/at_least_once' into add_test
make specs2 work
transaction interface refactor
Merge branch 'add_test' of https://github.com/manuzhang/gearpump into add_test
refactor transaction interface
transaction interface refactor