wal: pass struct instead of vclock to checkpoint methods

Currently, we only need to pass a vclock between TX and WAL during
checkpointing. However, in order to implement auto-checkpointing
triggered when WAL size exceeds a certain threshold, we will need to
pass some extra info so that we can properly reset the counter
accounting the WAL size in the WAL thread. To make it possible, let's
move wal_checkpoint struct, which is used internally by WAL to pass a
checkpoint vclock, to the header and require the caller to pass it to
wal_begin/commit_checkpoint instead of just a vclock.