Added generic header checks to bch_bkey_try_merge,
which then calls the bkey specific function

Removed extraneous checks from bch_extent_merge
Signed-off-by: NNicholas Swenson <nks@daterainc.com>

Now handling overlapping extents/keys is a method that's specific to what the
btree node contains.
Signed-off-by: NKent Overstreet <kmo@daterainc.com>

More work to disentangle various code from struct btree
Signed-off-by: NKent Overstreet <kmo@daterainc.com>

Soon, bset.c won't need to depend on struct btree.
Signed-off-by: NKent Overstreet <kmo@daterainc.com>

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