* route log buffer flush copies through the shared slab pool
Each flush copied the sealed window into a bytes.Buffer drawn from a
package-local sync.Pool. GC drains that pool, so once collections run
often (e.g. under GOMEMLIMIT) most flushes miss and Write grows a fresh
window-sized array, the dominant bytes.growSlice source under write load.
Copy into a size-classed slab from weed/util/mem instead. Slabs are
reused process-wide and returned to their exact size class after the
flush, so variable-sized flushes across many partitions stop churning
mismatched buffers.
* guard nil slab in flush releaseMemory
mem.Free(nil) resolves to the smallest slot pool and stores a zero-cap
slice, so a later mem.Allocate could return nil and panic. The current
call sites never pass nil, but the guard keeps a double release harmless.