Add support for the ABSTTL option of the RESTORE command.

[charettes]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Does travis tests pass with this change (enable it first in your forked repo and wait for the travis build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

Description of change

The ABSTTL option of the RESTORE command has been available since Redis 5.0. This PR adds an absttl=False kwarg to the restore command to allow enabling this option.

[codecov-io]

Codecov Report

Merging #1423 (5abe08a) into master (8c176cd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1423   +/-   ##
=======================================
  Coverage   86.58%   86.59%           
=======================================
  Files           6        6           
  Lines        2900     2902    +2     
=======================================
+ Hits         2511     2513    +2     
  Misses        389      389           

Impacted Files	Coverage Δ
redis/client.py	87.02% <100.00%> (+0.01%)	⬆️

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[charettes]

@bmerry all comments should be addressed.

[andymccurdy]

This looks great. Thanks!