This is a fix aimed to enable jsonwriter to checkpoint in accordance
with delta.checkpointInterval.  It changes the default commitbuilder to
set a post_commit_hook so that checkpointing will be done by default.
Potentially we could also expose CommitProperties as an argument to
flush_and_commit, but that would require a change to the function
signature and would be a breaking change.

Signed-off-by: Justin Jossick <[email protected]>