We have so many cursor types that it will be unbearable to properly make
all of them work. Let's simplify this and only focus on lazy cursor
which in the future will load from database in case we need it.
Add an explicit rewind() to move to the beginning. Change forward()
semantics so that *after* first forward() call, you are pointing to the
first row, which matches the get_next_record() semantics in B-tree
cursor.
The logging code that writes out transactions to disk needs to write out
the byte array that we actually use. The code is less hairly without the
generics so drop them.
