[converged] reduce `nng` thread counts

[catfact]

re:
#1706 (comment)

i've forked the nng repo under monome and noted what appear to be the relevant vars in nng/CMakeLists.txt

set(NNG_MAX_TASKQ_THREADS 16 CACHE STRING "Upper bound on taskq threads, 0 for no limit")

set(NNG_MAX_EXPIRE_THREADS 8 CACHE STRING "Upper bound on expire threads, 0 for no limit")

my wonderings:

• what would be a reasonable count guess? just one thread per task? (given our very simple 1peer <-> 1peer graph)
• can we trigger a cmake build from waf if we submodule nng under third-party? if not then i guess this might better be moved to norns-image

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also mostly out of curiousity: is having excess nng threads a significant / blocking issue? seems like if idle they wouldn't have too much impact