Fix flaky dist_api_calls test

[nikkhils]

The ts_dist_cmd_invoke_on_data_nodes_using_search_path() function cannot assume that ts_dist_cmd_invoke_on_data_nodes() function will return the SAME connection to any given DN across successive calls. A cache invalidation in the middle can cause another connection to be returned. This can cause unexpected errors.

We now fetch a list of connections to the specific datanodes in this function and re-use the same ones till the end of the function for all subsequent commands.

Fixes #4022

[codecov[bot]]

Codecov Report

Merging #4689 (9c2a430) into main (a26a597) will increase coverage by 0.01%. The diff coverage is 82.00%.

@@            Coverage Diff             @@
##             main    #4689      +/-   ##
==========================================
+ Coverage   90.82%   90.83%   +0.01%     
==========================================
  Files         224      224              
  Lines       42344    42391      +47     
==========================================
+ Hits        38457    38507      +50     
+ Misses       3887     3884       -3     

Impacted Files	Coverage Δ
tsl/src/remote/dist_commands.c	90.76% <82.00%> (-2.19%)	:arrow_down:
src/bgw/job.c	94.80% <0.00%> (+0.22%)	:arrow_up:
tsl/src/nodes/data_node_dispatch.c	96.72% <0.00%> (+0.23%)	:arrow_up:
src/bgw/scheduler.c	86.54% <0.00%> (+2.92%)	:arrow_up:

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[nikkhils]

this is being abandoned since @pmwkaa has a different approach to solve this in https://github.com/timescale/timescaledb/pull/4771