Mitigate torn writes for current time on 32-bit platforms

AfterAccessPolicy and DiscretePolicy both store the current time in a local Time variable to skip repeated calls to get the current time. On x86, this could result in a torn write/bad time value. Instead, use Interlocked read/write on 32bit.

Results are pretty close for x64, 3x penalty for x86 (this is still incredibly cheap, it's 3x slower than reading a register). This is still at least 2x faster than calling the function to get the time.

Method	Jit	Platform	Runtime	Mean	Error	StdDev	Ratio
TimeOriginal	RyuJit	X64	.NET 6.0	0.2273 ns	0.0351 ns	0.0360 ns	1.00
Time2	RyuJit	X64	.NET 6.0	0.2583 ns	0.0089 ns	0.0079 ns	1.16

TimeOriginal	RyuJit	X64	.NET Framework 4.8	0.0000 ns	0.0000 ns	0.0000 ns	?
Time2	RyuJit	X64	.NET Framework 4.8	0.0137 ns	0.0223 ns	0.0239 ns	?

TimeOriginal	LegacyJit	X86	.NET Framework 4.8	2.4370 ns	0.1943 ns	0.1817 ns	1.00
Time2	LegacyJit	X86	.NET Framework 4.8	7.8259 ns	0.1746 ns	0.1548 ns	3.25

Disassembly on X64 shows that assembly is identical for .NET6, and very similar for .NET 4.8, so the JIT is able to elide the branch and this does not introduce any overhead.

.NET 6.0.30 (6.0.3024.21525), X64 RyuJIT AVX2

; BitFaster.Caching.Benchmarks._32bitTImeTest.TimeOriginal()
;             return timeOrig.Last; 
;             ^^^^^^^^^^^^^^^^^^^^^
       mov       rax,26C40258DE0
       mov       rax,[rax]
       mov       rax,[rax+8]
       ret
; Total bytes of code 18

.NET 6.0.30 (6.0.3024.21525), X64 RyuJIT AVX2

; BitFaster.Caching.Benchmarks._32bitTImeTest.Time2()
;             return time.Last; 
;             ^^^^^^^^^^^^^^^^^
       mov       rax,1CCEE588DE8
       mov       rax,[rax]
       mov       rax,[rax+8]
       ret
; Total bytes of code 18

.NET Framework 4.8.1 (4.8.9241.0), X64 RyuJIT VectorSize=256

; BitFaster.Caching.Benchmarks._32bitTImeTest.TimeOriginal()
;             return timeOrig.Last; 
;             ^^^^^^^^^^^^^^^^^^^^^
       mov       rax,[141EBEC8]
       mov       rax,[rax+8]
       ret
; Total bytes of code 13

.NET Framework 4.8.1 (4.8.9241.0), X64 RyuJIT VectorSize=256

; BitFaster.Caching.Benchmarks._32bitTImeTest.Time2()
;             return time.Last; 
;             ^^^^^^^^^^^^^^^^^
       mov       rax,[13B2BED0]
       mov       edx,[rax]
       mov       rax,[rax+8]
       ret
; Total bytes of code 15

bitfaster / BitFaster.Caching

Mitigate torn writes for current time on 32-bit platforms #592