Reserve capacity when decoding HPACK headers

[glbrntt]

Motivation:

I was seeing frequent reallocations of the headers array as it
resized during decoding.

Modifications:

Reserve capacity for at least 16 HPACK headers during decoding.

Result:

Fewer reallocations at the cost of allocating more than we might need.

[Lukasa]

LGTM, seems like a nice change.

[weissi]

great job @glbrntt

from

DEBUG: [["remaining_allocations": 0, "total_allocations": 14000], ["remaining_allocations": 0, "total_allocations": 14000], ["remaining_allocations": 0, "total_allocations": 14000], ["remaining_allocations": 0, "total_allocations": 14000], ["total_allocations": 14000, "remaining_allocations": 0], ["remaining_allocations": 0, "total_allocations": 14000], ["total_allocations": 14000, "remaining_allocations": 0], ["total_allocations": 14000, "remaining_allocations": 0], ["total_allocations": 14000, "remaining_allocations": 0], ["total_allocations": 14000, "remaining_allocations": 0]]

to

DEBUG: [["total_allocations": 5000, "remaining_allocations": 0], ["total_allocations": 5000, "remaining_allocations": 0], ["total_allocations": 5000, "remaining_allocations": 0], ["remaining_allocations": 0, "total_allocations": 5000], ["total_allocations": 5000, "remaining_allocations": 0], ["total_allocations": 5000, "remaining_allocations": 0], ["total_allocations": 5000, "remaining_allocations": 0], ["total_allocations": 5000, "remaining_allocations": 0], ["remaining_allocations": 0, "total_allocations": 5000], ["total_allocations": 5000, "remaining_allocations": 0]]

so this goes from 14 allocations to 5 for 3 example HPACK decodes.

[weissi]

looks good to me, thanks George!