[dbnode] Raise rlimit to number of file limit by default

[robskillington]

What this PR does / why we need it:

Some users are setting all the kernel settings right, but they don't know to set the environment variable. It is not harmful to just attempt raising the limit, so that's what this change does (avoids requiring the environment variable to be set).

See for reference:

> Oh, looking from the pod, I do see the sysctl settings took, but it seems I'll need to find a way to get this to actually play nice:
/ # cat /proc/sys/fs/nr_open
3000000
/ # sysctl -n fs.nr_open
3000000
/ # sysctl -n fs.file-max
3000000
/ # cat /proc/self/limits | grep files
Max open files 65536 65536 files

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[martin-mao]

LGTM

[codecov]

Codecov Report

Merging #1792 into master will increase coverage by <.1%.
The diff coverage is 40%.

@@           Coverage Diff            @@
##           master   #1792     +/-   ##
========================================
+ Coverage      72%   72.1%   +<.1%     
========================================
  Files         986     986             
  Lines       82876   82875      -1     
========================================
+ Hits        59742   59755     +13     
+ Misses      19195   19179     -16     
- Partials     3939    3941      +2

Flag	Coverage Δ
#aggregator	82.4% <ø> (ø)	⬆️
#cluster	85.7% <ø> (ø)	⬆️
#collector	63.9% <ø> (ø)	⬆️
#dbnode	80.1% <40%> (ø)	⬆️
#m3em	73.2% <ø> (ø)	⬆️
#m3ninx	74.1% <ø> (ø)	⬆️
#m3nsch	51.1% <ø> (ø)	⬆️
#metrics	17.5% <ø> (ø)	⬆️
#msg	74.7% <ø> (ø)	⬆️
#query	66.6% <ø> (ø)	⬆️
#x	85.2% <ø> (+0.1%)	⬆️

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