rlp: avoid list header allocation in encoder (#21274)

List headers made up 11% of all allocations during sync. This change
removes most of those allocations by keeping the list header values
cached in the encoder buffer instead. Since encoder buffers are pooled,
list headers are no longer allocated in the common case where an
encoder buffer is available for reuse.

Co-authored-by: Felix Lange <fjl@twurst.com>
# Conflicts:
#	rlp/encode.go