Optimize tarfile uncompression performance

[methane]

BPO	34043
Nosy	@gustaebel, @methane, @serhiy-storchaka
PRs	bpo-34043: Optimize tarfile uncompress performance #8089

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-07-06.07:31:24.312>
created_at = <Date 2018-07-04.12:10:32.516>
labels = ['3.8', 'library', 'performance']
title = 'Optimize tarfile uncompression performance'
updated_at = <Date 2018-07-06.07:31:24.311>
user = 'https://github.com/methane'

bugs.python.org fields:

activity = <Date 2018-07-06.07:31:24.311>
actor = 'methane'
assignee = 'none'
closed = True
closed_date = <Date 2018-07-06.07:31:24.312>
closer = 'methane'
components = ['Library (Lib)']
creation = <Date 2018-07-04.12:10:32.516>
creator = 'methane'
dependencies = []
files = []
hgrepos = []
issue_num = 34043
keywords = ['patch']
message_count = 2.0
messages = ['321039', '321148']
nosy_count = 3.0
nosy_names = ['lars.gustaebel', 'methane', 'serhiy.storchaka']
pr_nums = ['8089']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue34043'
versions = ['Python 3.8']

[methane]

tarfile._Stream has two buffer for compressed and uncompressed data.
Those buffers are not aligned so unnecessary bytes slicing happens
for every reading chunks.

This commit bypass compressed buffering.

In this benchmark 1, user time become 250ms from 300ms.

[methane]

New changeset 8d13091 by INADA Naoki in branch 'master':
bpo-34043: Optimize tarfile uncompress performance (GH-8089)
8d13091