bcache: Convert bch_btree_read_async() to bch_btree_map_keys()

This is a fairly straightforward conversion, mostly reshuffling -
op->lookup_done goes away, replaced by MAP_DONE/MAP_CONTINUE. And the
code for handling cache hits and misses wasn't really btree code, so it
gets moved to request.c.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

diff --cc drivers/md/bcache/btree.c
Simple merge

diff --cc drivers/md/bcache/btree.h
Simple merge

diff --cc drivers/md/bcache/journal.c
Simple merge

diff --cc drivers/md/bcache/request.c
Simple merge

diff --cc drivers/md/bcache/request.h
Simple merge