Tribune bulk adds

Issue: 1. of #68 (closed)

Introduce following functions:

Persist.bulk_adds : (key * value) list -> t -> t
Persist_plus.bulk_adds : (key * value) list -> t -> t

Invariants:

• The first argument kvs must be sorted with the order of keys
• For all keys k in the second argument node,　k must be either less than all keys in kvs or greater than all keys in kvs.

Testings

• tribune/test/test_persist_plus.ml : test_bulk_adds