MB-7882 Optimize btree node decoding

Using the +bin_opt_info flag, the Erlang compiler told the
decode function was not binary optimized:

/opt/r14b04/bin/erlc -I../../src +bin_opt_info  couch_btree.erl;
./couch_btree.erl:70: Warning: NOT OPTIMIZED: sub binary used by binary:copy/1
./couch_btree.erl:433: Warning: NOT OPTIMIZED: called function decode_node/3 does not begin with a suitable binary matching instruction
./couch_btree.erl:442: Warning: NOT OPTIMIZED: called function decode_node/3 does not begin with a suitable binary matching instruction

After this change, the compiler tells it is now optimized:

/opt/r14b04/bin/erlc -I../../src +bin_opt_info  couch_btree.erl;
./couch_btree.erl:73: Warning: NOT OPTIMIZED: sub binary used by binary:copy/1
./couch_btree.erl:436: Warning: OPTIMIZED: creation of sub binary delayed
./couch_btree.erl:447: Warning: OPTIMIZED: creation of sub binary delayed

Change-Id: Ie14a85a76a4e00f1f6aac99e19218fbd9841022c
Reviewed-on: http://review.couchbase.org/25394
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>