Replacing usage of ManualResetEvent with ManualResetEventSlim

[stebet]

Proposed Changes

Since the client never does cross-appdomain or cross-process communication internally, there is no need to use ManualResetEvent instead of ManualResetEventSlim.

ManualResetEventSlim can often avoid allocating a kernel-level object by using a spinlock wait mechanism for a short duration before switching so it can often stay inside the managed code boundaries.

See detailed description and performance characteristics here: https://download.microsoft.com/download/B/C/F/BCFD4868-1354-45E3-B71B-B851CD78733D/PerformanceCharacteristicsOfSyncPrimitives.pdf

Types of Changes

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

[michaelklishin]

I see no downsides to switching. Thank you for your time @stebet 👍

[michaelklishin]

Backported to 6.x.