add parameter for limiting maximum AOF files size on disk

[kronwerk]

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 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 70.90%. Comparing base (1acf7f7) to head (b57409c).

Files with missing lines	Patch %	Lines
src/aof.c	85.71%	1 Missing ⚠️
src/server.c	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1425      +/-   ##
============================================
+ Coverage     70.71%   70.90%   +0.19%     
============================================
  Files           119      119              
  Lines         64652    64625      -27     
============================================
+ Hits          45717    45822     +105     
+ Misses        18935    18803     -132     

Files with missing lines	Coverage Δ
src/config.c	77.59% <ø> (-0.06%)	⬇️
src/server.h	100.00% <ø> (ø)
src/aof.c	81.25% <85.71%> (+1.12%)	⬆️
src/server.c	87.48% <83.33%> (-0.09%)	⬇️

... and 43 files with indirect coverage changes