make vnode to get_fsm communication avoid a binary_to_term/term_to_binary roundtrip

This optimizes communication from vnodes to the get_fsm, by not doing binary_to_term on the vnode when executing a read, but rather make that happen in the get_fsm.  By doing so, we avoid an extra term_to_binary+binary_to_term imposed by erlang distribution.  It looks pretty silly that vnode gets the binary, turns it into a term and immediately hands it off to erlang distribution (which will then encode it again) for wire transfer.

I don't have hard data on how much CPU we save on this, but our performance measurements indicate that binary_to_term and term_to_binary are hot spots.

I'm not sure this is the right way to do this; and it certainly makes get_fsm/vnode interactions a tad more complicated, so consider this a proposal.