Unidirectional transports: optimizations and delta encoding support enabler

[FZambia]

Currently unidirectional transports require an intermediary ConnectRequest construction. Which abstracts away protocol package, but to avoid extra allocations this PR provides a way to use protocol.ConnectRequest directly when calling Client.ProtocolConnect. Also, support headers in intermediary ConnectRequest.

Also, optimizations for SSE and HTTP-streaming emulation transports: write many messages before calling Flush.

And open a road for delta compression for server-side subscriptions by setting Delta in subscriptions of ConnectRequest.

[codecov]

Codecov Report

Attention: Patch coverage is 64.19753% with 29 lines in your changes missing coverage. Please review.

Project coverage is 83.30%. Comparing base (e280f73) to head (cd3ff94).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
handler_http_stream.go	52.00%	8 Missing and 4 partials ⚠️
handler_sse.go	43.75%	6 Missing and 3 partials ⚠️
client.go	79.48%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
- Coverage   83.31%   83.30%   -0.02%     
==========================================
  Files          39       39              
  Lines        8840     8851      +11     
==========================================
+ Hits         7365     7373       +8     
- Misses       1118     1119       +1     
- Partials      357      359       +2     

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