daemon/session2: optimize allocations

daemon/session2: optimize allocations

A) Context-specific allocations
-------------------------------

There were two problems:

1) Some payloads are short-lived (e.g. allocated on stack) and we need
   to make a copy of them if the iteration over protocol layers becomes
   asynchronous.
2) The `pl_dns_stream_wrap` function used a mempool belonging to its
   session-wide context. Some sessions may live for a long time, which
   could potentially lead to needlessly long-lived memory allocations.

Both of these problems are solved in this commit by using a new
`knot_mm_t pool` field in `struct protolayer_iter_ctx`, which lives only
for a single submit (and survives asynchronicity). The whole pool is
then freed all at once when the `struct protolayer_iter_ctx` is
finalized.

B) Merging `struct protolayer_manager` into `struct session2`
-------------------------------------------------------------

It actually made no real sense to have the two separated. It only
introduced an extra layer of indirection and many layers actually needed
to access both anyway. This should simplify things considerably.