firebase-ios-sdk

Add `ToString()` to `LevelDbTransaction` (#946)

* Initial plumbing for leveldb local parts

* Add model::BatchId

* Port leveldb_key.{h,cc}

* Add string StartsWith

* Add leveldb_key_test.cc to the project

* Revert back to using leveldb::Slice for read/describe

These operations universally operate on keys obtained from leveldb so
it's actually unhelpful to force all the callers to make
absl::string_views from them.

* Start work on ToString for transactions

* Add ToString() method to LevelDbTransaction

* Style

* lint

* Revert addition of util::StartsWith