Previously, entry verification had a dedicated threadpool used to verify
PoH hashes as well as some basic transaction verification via
Bank::verify_transaction(). It should also be noted that the entry
verification code provides logic to offload to a GPU if one is present.
Regardless of whether a GPU is present or not, some of the verification
must be done on a CPU. Moreso, the CPU verification of entries and
transaction execution are serial operations; entry verification finishes
first before moving onto transaction execution.
So, tx execution and entry verification are not competing for CPU cycles
at the same time and can use the same pool.
One exception to the above statement is that if someone is using the
feature to replay forks in parallel, then hypothetically, different
forks may end up competing for the same resources at the same time.
However, that is already true given that we had pools that were shared
between replay of multiple forks. So, this change doesn't really change
much for that case, but will reduce overhead in the single fork case
which is the vast majority of the time.
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