Replace System.Random with Xorshift RNG.

[jlao]

A good chunk of time was spent in System.Random when we had to do evictions. Swapping out System.Random with a Xorshift RNG.

https://en.wikipedia.org/wiki/Xorshift

RngPerf benchmark shows Xorshift is quite a bit faster.

BenchmarkDotNet=v0.11.4, OS=Windows 10.0.17763.316 (1809/October2018Update/Redstone5)
Intel Core i9-9900K CPU 3.60GHz, 1 CPU, 16 logical and 8 physical cores
.NET Core SDK=2.1.504
  [Host] : .NET Core 2.1.8 (CoreCLR 4.6.27317.03, CoreFX 4.6.27317.03), 64bit RyuJIT
  Core   : .NET Core 2.1.8 (CoreCLR 4.6.27317.03, CoreFX 4.6.27317.03), 64bit RyuJIT

Job=Core  Runtime=Core  

Method	Mean	Error	StdDev	Ratio	Rank
Random	8.779 ns	0.1190 ns	0.1113 ns	1.00	2
XortShiftRandom	1.116 ns	0.0069 ns	0.0064 ns	0.13	1

Also better performance in HashCollision benchmark which forces evictions.

Before

Method	N	Mean	Error	StdDev	Rank
CacheTable	32	950.3 ns	2.011 ns	1.570 ns	1
CacheTable	64	1,876.8 ns	4.252 ns	3.769 ns	2

After

Method	N	Mean	Error	StdDev	Rank
CacheTable	32	883.7 ns	2.390 ns	2.235 ns	1
CacheTable	64	1,740.6 ns	8.394 ns	7.852 ns	2