In `async` functions, a closure environment is created for variables
that cross an await boundary - this closure environment is kept in
memory for the lifetime of the associated future - this means that
although _some_ variables are no longer used, they still take up memory
for a long time.
In Nimbus, message validation is processed in batches meaning the future
of an incoming gossip message stays around for quite a while - this
leads to memory consumption peaks of 100-200 mb when there are many
attestations in the pipeline.
To avoid excessive memory usage, it's generally better to move non-async
code into proc's such that the variables therein can be released earlier
- this includes the many hidden variables introduced by macro and
template expansion (ie chronicles that does expensive exception
handling)
* move seen table salt to floodsub, use there as well
* shorten seen table salt to size of hash
* avoid unnecessary memory allocations and copies in a few places
* factor out message scoring
* avoid reencoding outgoing message for every peer
* keep checking validators until reject (in case there's both reject and
ignore)
* `readOnce` avoids `readExactly` overhead for single-byte read
* genericAssign -> assign2
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