fetch: add streaming mode

[ivan4th]

Motivation

Excess buffering during sync causes unneeded GC pressure and sometimes leads to out-of-memory errors.

Description

Added streaming mode for the fetcher (feature-gated).

In streaming mode, no extra buffering is used for fetch server and client. Fetch client still uses some big slices such as list of ATX IDs in an epoch, but that's to be improved in a followup.

Streaming mode is enabled via fetch.streaming boolean config key, which defaults to false.

Test Plan

Verified sync on the testnet.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 79.42446% with 143 lines in your changes are missing coverage. Please review.

Project coverage is 80.0%. Comparing base (b09651a) to head (da777b2). Report is 3 commits behind head on develop.

Files	Patch %	Lines
fetch/handler.go	64.0%	27 Missing and 19 partials :warning:
fetch/fetch.go	77.1%	19 Missing and 8 partials :warning:
p2p/server/server.go	73.1%	19 Missing and 7 partials :warning:
codec/codec.go	52.9%	7 Missing and 1 partial :warning:
datastore/store.go	83.3%	6 Missing and 2 partials :warning:
fetch/mesh_data.go	89.7%	4 Missing and 3 partials :warning:
sql/database.go	91.4%	2 Missing and 2 partials :warning:
sql/identities/identities.go	86.2%	2 Missing and 2 partials :warning:
sql/layers/layers.go	88.2%	2 Missing and 2 partials :warning:
activation/activation.go	50.0%	0 Missing and 2 partials :warning:
... and 4 more

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5562     +/-   ##
=========================================
+ Coverage     79.8%   80.0%   +0.2%     
=========================================
  Files          279     279             
  Lines        28549   29009    +460     
=========================================
+ Hits         22789   23223    +434     
+ Misses        4186    4184      -2     
- Partials      1574    1602     +28     

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[ivan4th]

(almost forgot) Will further update this PR to move some of the codec stuff to go-scale, otherwise good for review

[ivan4th]

bors try

[spacemesh-bors[bot]]

try

Build failed:

• ci-status

[ivan4th]

bors try

[spacemesh-bors[bot]]

try

Build succeeded:

• systest-status
• ci-status

[ivan4th]

bors merge

[ivan4th]

bors cancel

[spacemesh-bors[bot]]

Canceled.

[ivan4th]

bors merge

[spacemesh-bors[bot]]

Pull request successfully merged into develop.

Build succeeded:

• ci-status
• systest-status