Investigate why time to complete 'zcbenchmark validatelargetx' has increased from z7 to z8

[bitcartel]

Per comments on Slack, time has increased 25-30% to complete test.
https://speed.z.cash/changes/?rev=95277e0311&exe=undefined&env=1

[bitcartel]

Two issues identified so far:

1. The test does not measure solely the verification of a single large transaction, but also the time to create (the same) CTransaction object thousands of times.
2. The time to create a CTransaction object has increased slightly from z7 to z8 due to the need to compute both the serialization hash (used as an object identifier by equality operators) and the non-malleable txid.

Issue 1 is resolved by PR #1194, which shows a reduction in time to complete the verification phase of the test by 75%+, where the z7 and z8 benchmarks are the same i.e. non-malleable txids do not affect signature verification times.

Regarding issue 2, there may be optimizations possible, such as on-demand generation and cacheing of the txid, for situations like this test where CTransactions are created but there is no intention of asking it for its txid. This can be placed into a new ticket for further research.