add parameter for limiting maximum AOF files size on disk

[kronwerk]

[duplicate of this]

one can optionally want to limit AOF disk usage not with disk real size, but with some tailored value. this PR makes it possible. possible use cases:

1. filling disk up with AOF to the real limit may break something in user's environment (logs, watchdogs, anything) - setting lower value prevents that completely;
2. setting this new limit to some low value may allow automatic solving of filling disk problem for those users who don't care too much about disk cost:

• intense writes may overcome automatic AOF rewrite with some user's settings at some moment;
• AOF reach lower than full disk limit (e.g. 50-60% of disk);
• eventually automatic AOF rewrite comes in once more and finally fixes that. while on 100% filled disk (like nowadays) that automatic solution is impossible due to insufficient space for tmp AOF rewrite files (0 space, actually) - one should size up disk under running database which makes some people a bit nervous, as I noticed, and in some cases that's not always convenient or even possible.

Fixes #540

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 71.44%. Comparing base (663dac9) to head (274cba6).

Files with missing lines	Patch %	Lines
src/aof.c	70.00%	3 Missing :warning:
src/server.c	90.00%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1630      +/-   ##
============================================
+ Coverage     71.30%   71.44%   +0.14%     
============================================
  Files           123      123              
  Lines         67122    67135      +13     
============================================
+ Hits          47859    47966     +107     
+ Misses        19263    19169      -94     

Files with missing lines	Coverage Δ
src/config.c	78.47% <ø> (ø)
src/server.h	100.00% <ø> (ø)
src/server.c	88.09% <90.00%> (-0.01%)	:arrow_down:
src/aof.c	80.38% <70.00%> (-0.15%)	:arrow_down:

... and 11 files with indirect coverage changes

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[kronwerk]

@xbasel @soloestoy pls take a look, I've dumped my previous similar PR where you were reviewers