Handle object-oriented code/polymorphism correctly

There are several places in the code where there is potentially unsafe casting in order to implement object-oriented code and polymorphism. The correct way to handle this is to create virtual translation functions.

Implements correctly:
clockcache.c

Implements safely, but incorrectly (e.g. merge_advance should take a merge_iterator and there should be a virtual wrapper function which performs the cast):
merge.c
btree.c (btree_iterator)
laio.c
shard_log.c
splinter.c

Implements unsafely:
rc_allocator.c

Start by studying the virtual translation approach already implemented in clockcache.c. Then audit merge.c, btree.c, laio.c, shard_log.c, splinter.c, and rc_allocator.c, with particular attention to merge_advance and btree_iterator; done means the unsafe casts are replaced with correctly typed virtual wrapper functions.