For the case of gzip included in plain response, the esi_level == 1 vdp
order was:

	pesi gunzip pesi_buf V2P(to_parent)

Yet we had assertions in place that pesi_buf always immediately follows
pesi.

The reason was that, for esi_level > 0, we would not push pesi_buf from
pesi init but rather from the transport, which was plain wrong: We
should delay any additional vdps in order to buffer the least amount of
data.

Working on this, I also noted that for the generic buffering case, our
assertion that pesi_buf is first, might be too strict. Now, any VDPs
before the buffer are being closed at esi_level > 1.

fixes this panic:

Assert error in vped_close_vdp(), vdp_pesi.c line 1182:
  Condition(vdpe->vdp == vdp) not true.

	...

Backtrace:
  0x43cf3b: /usr/sbin/varnishd() [0x43cf3b]
  0x4a01c2: /usr/sbin/varnishd(VAS_Fail+0x42) [0x4a01c2]
  0x7f3719306b63:
./vmod_cache/_vmod_pesi.4d9e0603bac2a2e2b2627f7fe90ff1d55d4759545517c869a5571f16636e230e(+0x8b63)
[0x7f3719306b63]
  0x7f371930a377:
./vmod_cache/_vmod_pesi.4d9e0603bac2a2e2b2627f7fe90ff1d55d4759545517c869a5571f16636e230e(+0xc377)
[0x7f371930a377]
  0x441eab: /usr/sbin/varnishd(CNT_Request+0x11ab) [0x441eab]
  0x7f3719308043:
./vmod_cache/_vmod_pesi.4d9e0603bac2a2e2b2627f7fe90ff1d55d4759545517c869a5571f16636e230e(+0xa043)
[0x7f3719308043]
  0x45c833: /usr/sbin/varnishd() [0x45c833]
  0x45ccf0: /usr/sbin/varnishd() [0x45ccf0]
  0x7f37e6d18e65: /lib64/libpthread.so.0(+0x7e65) [0x7f37e6d18e65]
  0x7f37e6a4188d: /lib64/libc.so.6(clone+0x6d) [0x7f37e6a4188d]
thread = (cache-worker)
pthread.attr = {
  guard = 4096,
  stack_bottom = 0x7f372c482000,
  stack_top = 0x7f372c502000,
  stack_size = 524288,
}
thr.req = 0x7f3601106020 {
  vxid = 25559391, transport = PESI_INCLUDE
  step = R_STP_TRANSMIT,
  req_body = R_BODY_NONE,
  restarts = 0, esi_level = 1,

	...