Btree updates before we go RW work by inserting into the array of keys
that journal replay will insert - but inserting into a flat array is
O(n), meaning if btree_gc needs to update many alloc keys, we're O(n^2).

Fortunately, the updates btree_gc does happens in sequential order,
which means a gap buffer works nicely here - this patch implements a gap
buffer for journal keys.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>