mpstream: get rid of mpstream_reset

Proposed ASAN implementation of region allocator does not support double
reservation for the sake of simplicity. Every reservation is supposed to
be followed by one or more allocations.

This restiction does not work well with mpstream currently. The issue is
mpstream_init/mpstream_reserve do reservation of size 0. For example In
case of region slab of min order is reserved (a chunk of memory of page
size currently). If the first data we want to write to mpstream is
larger then the reservation done then we make reservation again.

Let's get rid of this reservation at the beginning as it is suboptimal
behaviour. Moreover let's get rid of mpstream_reset as mpstream_init
is lightweight and we can create a new mpstream instead of reusing
exiting.

Also while we at it avoid allocation of 0 size in mpstream_flush as it
is done in mpstream_reserve_slow (see 3.0.0-alpha3-19-g8159347d0 "misc:
avoid allocations of size 0 for region" for details).

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