Make log output always line-buffered

[gumb0]

Grid team reports that when reading log output from a client process through a pipe, other clients make it line-buffered (non-default behavior for stdout, which is line-buffered only when attached to tty)

Here I enable flushing after each line in boost.log.

The difference is noticeable when running commands like

>aleth/aleth >aleth.log &
>tail -f aleth.log

cc @evertonfraga

[evertonfraga]

amazing, @gumb0. thanks a lot!

[codecov-io]

Codecov Report

Merging #5589 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5589      +/-   ##
==========================================
- Coverage   62.23%   62.21%   -0.02%     
==========================================
  Files         347      347              
  Lines       29136    29137       +1     
  Branches     3295     3295              
==========================================
- Hits        18132    18129       -3     
- Misses       9826     9830       +4     
  Partials     1178     1178

[evertonfraga]

I just made a build of aleth and it worked as intended. waiting for the RC release ;)