Throttle the number of pending txns in WriteBatch

[manishrjain]

Currently, WriteBatch can create too many pending txns, which can cause memory issues. Instead, we should add a method SetMaxPending(max int) (default to 100 maybe), which can be enforced via Commit and callback using Throttle. In fact, Throttle can be used to simplify the internals of WriteBatch.

[manishrjain]

We also need Throttle in db.Load method for similar reasons.

[tsellers-r7]

As a note, this looks related to the other P0 Issue
#714

[ashish-goswami]

Fixed in #762