Increase rioConnsetWrite max chunk size to 16K

[ranshid]

Fixes #796

Currently rioConnWite uses 1024 bytes chunk when feeding the replication sockets on RDB write. This value seems too small and we will end up with high syscall overhead.
This PR sets the max chunk size to 16K.

Using a simple test program we did not observe any significant improvement in read/write times going with chunks bigger than 4K but that might be la bottleneck on network throughput. We did observe sweet point of CPU utilization at 16K when using TLS.

Test
test-program.zip
Include client/server implementations for client sending 100MB data to a server. the tests are both in TLS and TCP.
I run the tests on separate m5.xlarge EC2 instances on AWS.
the executables were compiled and run on ubuntu:

lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 24.04 LTS
Release:	24.04
Codename:	noble

uname -a
Linux ip-172-31-22-140 6.8.0-1009-aws #9-Ubuntu SMP Fri May 17 14:39:23 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

All files were compiled with O3 optimization level.

gcc --version
gcc (Ubuntu 13.2.0-23ubuntu4) 13.2.0

Results:

<style> </style>

Chunk Size	write-time sec	writes total	write cpu-time (usr+sys)	read-time sec	read total syscalls	read cpu-time (usr+sys)
1K	0.162946	102400	0.185916	0.168479	2447	0.026945
4K	0.163036	25600	0.122629	0.168627	715	0.023382
8K	0.163942	12800	0.121131	0.168887	704	0.039388
16K	0.163614	6400	0.104742	0.168202	2483	0.025574
64K	0.16279	1600	0.098792	0.168854	1068	0.046929
1K - TLS	0.32648	102400	0.366961	0.330785	102400	0.337377
4K - TLS	0.164296	25600	0.183326	0.169032	25600	0.129952
8K - TLS	0.163977	12800	0.163118	0.169484	12800	0.098432
16K - TLS	0.164861	6400	0.150666	0.169878	6383	0.094794
64K - TLS	0.163704	6400	0.156125	0.169323	6388	0.089971

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.37%. Comparing base (6eb19cf) to head (b898413).
Report is 7 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #817      +/-   ##
============================================
+ Coverage     70.32%   70.37%   +0.04%     
============================================
  Files           112      112              
  Lines         61269    61310      +41     
============================================
+ Hits          43089    43144      +55     
+ Misses        18180    18166      -14     

Files	Coverage Δ
src/rio.c	84.18% <100.00%> (ø)
src/server.h	100.00% <ø> (ø)

... and 17 files with indirect coverage changes

[madolson]

You reference fdset, which is the old version before we added the connection abstraction. I think this change makes sense though.