Reduce closure allocations

[mtmk]

This pull request focuses on improving timer initialization and timeout handling in the NatsSubBase class, and introduces a new microbenchmark for subscription construction. The main changes ensure that timers are only created when necessary and initialized with explicit parameters, and that timeouts default to TimeSpan.Zero instead of the default struct value. Additionally, the code now uses static delegates for timer callbacks to reduce allocations.

Timeout and Timer Initialization Improvements:

• Changed the default values for _idleTimeout, _startUpTimeout, and _timeout to use TimeSpan.Zero instead of the default struct value, ensuring more explicit and predictable timeout behavior.
• Updated timer creation logic to check for TimeSpan.Zero instead of default and to initialize timers with explicit state and callback parameters. This reduces GC pressure and ensures timers are only allocated when needed. [1] [2]

Delegate and Callback Optimization:

• Replaced inline delegates for cancellation token and timer callbacks with static delegates, reducing allocations and improving performance. [1] [2]

Benchmarking:

• Added a new microbenchmark class SubscribeCtor to measure the performance of constructing new NatsSub<int> instances, aiding in performance testing and analysis.

Benchmark SubscribeCtor.NewNatsSub

this PR:

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
NewNatsSub	2.270 us	1.555 us	0.0852 us	0.1183	0.1144	1.61 KB

main:

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
NewNatsSub	2.790 us	0.7886 us	0.0432 us	0.1335	0.1297	1.8 KB

[scottf]

LGTM