Add WriteBatch type to KvStore

[MarkJr94]

Problem

To complete integration, needs to add a WriteBatch type implementing the WriteBatch [1] interface used by blocktree. This WriteBatch will also need shared access to the write-ahead log.

The Database trait is also missing a fn commit(&self, batch: Self::WriteBatch) -> Result<()> method [2].

Proposed Solution

Implement a WriteBatch type, integrate it with the store.

'Lift' the write-ahead log directly into the KvStore, put it behind an Arc and give a clone to each WriteBatch.

Add method to to trait

[1]

solana/core/src/blocktree/db.rs

Lines 50 to 52 in 9fac3b2

	pub trait IWriteBatch<D: Database> {
	fn put_cf(&mut self, cf: D::ColumnFamily, key: &D::KeyRef, data: &[u8]) -> Result<()>;
	}

[2]

solana/core/src/blocktree/db.rs

Lines 12 to 34 in 9fac3b2

	pub trait Database: Sized + Send + Sync {
	type Error: Into<Error>;
	type Key: Borrow<Self::KeyRef>;
	type KeyRef: ?Sized;
	type ColumnFamily;
	type Cursor: Cursor<Self>;
	type EntryIter: Iterator<Item = Entry>;
	type WriteBatch: IWriteBatch<Self>;
	fn cf_handle(&self, cf: &str) -> Option<Self::ColumnFamily>;
	fn get_cf(&self, cf: Self::ColumnFamily, key: &Self::KeyRef) -> Result<Option<Vec<u8>>>;
	fn put_cf(&self, cf: Self::ColumnFamily, key: &Self::KeyRef, data: &[u8]) -> Result<()>;
	fn delete_cf(&self, cf: Self::ColumnFamily, key: &Self::KeyRef) -> Result<()>;
	fn raw_iterator_cf(&self, cf: Self::ColumnFamily) -> Result<Self::Cursor>;
	fn write(&self, batch: Self::WriteBatch) -> Result<()>;
	fn batch(&self) -> Result<Self::WriteBatch>;
	}