Dave Collins 6b802379ec txscript: Shallow tx copy for signature hash calc. ...

This modifies calcSignatureHash to use a shallow copy of the transaction
versus a deep copy since the actual scripts themselves are not modified
and therefore don't need to be copied.

This is being done because profiling the most overall allocated space
shows that the deep copy performed in calcSignatureHash accounts for
nearly 20% of all allocations on a synced running instance.  Also,
copying all of the additional data makes it more time consuming as well.

With this change, that figure drops from ~20% to ~5% of all allocations.

The following benchmark shows the relative speedups and allocation
reduction as a result of the optimization on my system.  In particular,
the changes result in approximately a 15% speedup and a whopping 99.89%
reduction in allocations when using a large transaction with thousands
of inputs which was the worst case scenario.

benchmark                        old allocs    new allocs    delta
--------------------------------------------------------------------
BenchmarkCalcSignatureHash       11151         12            -99.89%

benchmark                        old ns/op     new ns/op     delta
--------------------------------------------------------------------
BenchmarkCalcSignatureHash       3599845       3056359       -15.10%

2017-08-24 12:48:11 -05:00

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