This commit adds checksums to the meta pages on every write. When the
database loads, it verifies the checksums on the meta pages and returns
an error if either one is corrupt.
In the future, it should fallback to the previous meta page but for right
now it just hard fails. This is at least preferable to opening the database
and getting a random error or further corruption.
Fixes#25.
I changed the Transaction/RWTransaction types to Tx/RWTx, respectively. This makes the naming
more consistent with other packages such as database/sql. The txnid is changed to txid as well.