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]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 70.99%. Comparing base (a18fcdb) to head (2954eb0).
Report is 2 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/aof.c	87.50%	1 Missing ⚠️
src/server.c	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1630      +/-   ##
============================================
+ Coverage     70.84%   70.99%   +0.14%     
============================================
  Files           121      121              
  Lines         65169    65180      +11     
============================================
+ Hits          46172    46274     +102     
+ Misses        18997    18906      -91     

Files with missing lines	Coverage Δ
src/config.c	78.39% <ø> (ø)
src/script.c	87.69% <ø> (ø)
src/server.h	100.00% <ø> (ø)
src/aof.c	81.37% <87.50%> (+1.13%)	⬆️
src/server.c	87.64% <87.50%> (+0.02%)	⬆️

... and 11 files with indirect coverage changes

[kronwerk]

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