Synchronized buffer

[wpbrown]

• added a new function to servicecall to allow only advancing to the service call and then returning the service response future
• buffer can maintain order/backpressure by implementing strict readiness and synchronous calling
• buffer can flush in order or cancel pending buffered futures on shutdown

[codecov]

Codecov Report

Patch coverage: 63.04% and project coverage change: -0.14 ⚠️

Comparison is base (4380b3a) 90.99% compared to head (218cedd) 90.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
- Coverage   90.99%   90.86%   -0.14%     
==========================================
  Files         189      189              
  Lines       30829    30944     +115     
==========================================
+ Hits        28052    28116      +64     
- Misses       2777     2828      +51     

Impacted Files	Coverage Δ
ntex-service/src/lib.rs	79.62% <ø> (ø)
ntex-util/src/services/buffer.rs	78.26% <56.43%> (-20.97%)	⬇️
ntex-service/src/ctx.rs	92.18% <81.08%> (-2.66%)	⬇️

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