Initial attempt at h2/DB storage implementation (LockStore only)

Lays down the groundwork for h2/MyBatis integration in the
aurora scheduler, and creates a bridge so that the existing
in-memory stores can co-exist with the new db stores. This
allows us to incrementally replace each storage implementation in
org.apache.aurora.scheduler.storage.mem. In this patch I have replaced
MemLockStore with DbLockStore.

Testing Done:
./gradlew clean build

Bugs closed: AURORA-335

Reviewed at https://reviews.apache.org/r/21132/