Add maxmemory-reserved-scale parameter for evicting key earlier to avoid OOM

[hwware]

Reference: #742 and https://learn.microsoft.com/en-us/azure/azure-cache-for-redis/cache-best-practices-memory-management (Azure)

Generally, when clients set maxmemory-policy as allkeys-lru or other memory eviction policies, and maxmemory as well, If server runs as write-heavy workloads, the data stored in memory could reach the maxmemory limit very quickly, then OOM message will be reported.

If we have maxmemory-reserved-scale parameter and client enable the lazyfree-lazy feature, for example, we can set maxmemory-reserved-scale as 0.8, it means key eviction begin when used memory reaches 8GB if maxmemory is 10GB, thus, server could continue process clients data and OOM will not happen at this time.

Thus, we can see the benefit is we can delay OOM time, but we need pay for less memory available.

One example for this paramter：
Assume

maxmemory 4GB
maxmemory-reserved-scale 20

Then we could check the detail by info memory command:

maxmemory:4294967296
maxmemory_human:4.00G
maxmemory_policy:allkeys-lru
maxmemory_reserved_scale:20
maxmemory_available:3435973836
maxmemory_available_human:3.20G

We could also update and get the maxmemory-reserved-scale value during runtime as following:

config set maxmemory-reserved-scale value
config get maxmemory-reserved-scale

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 16 lines in your changes missing coverage. Please review.

Project coverage is 70.57%. Comparing base (9f8185f) to head (0a68013).

Files with missing lines	Patch %	Lines
src/config.c	18.18%	9 Missing ⚠️
src/evict.c	56.25%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #831      +/-   ##
============================================
- Coverage     70.59%   70.57%   -0.03%     
============================================
  Files           114      114              
  Lines         61670    61697      +27     
============================================
+ Hits          43538    43544       +6     
- Misses        18132    18153      +21     

Files with missing lines	Coverage Δ
src/server.c	88.61% <100.00%> (+0.01%)	⬆️
src/server.h	100.00% <ø> (ø)
src/evict.c	95.63% <56.25%> (-3.23%)	⬇️
src/config.c	78.26% <18.18%> (-0.44%)	⬇️

... and 13 files with indirect coverage changes

[enjoy-binbin]

this seem like a interesting feature, did not review, just drop a comment that approve the concept.

[madolson]

I also like the idea. I just haven't really spent enough time thinking about it. Memory management is a big area in Valkey we need to improve.