Refactor gateway kernel management to achieve a degree of consistency

[kevin-bates]

Issue #455 correctly points out an inconsistency between the GatewayKernelManager vs the other MappingKernelManager instances within the server in that its get_kernel() method doesn't return a KernelManager instance, but rather a JSON object which corresponds to the model returned from the configured Gateway Server that corresponds the targeted kernel manager.

This pull request introduces both a GatewayKernelManager (renaming the previous GKM to GatewayMappingKernelManager) and a GatewayKernelClient class that essentially proxy all requests (and responses) to (from) the configured Gateway Server instance.

It should be noted that these proxy objects do not support the full functionality of regular KernelManager and KernelClient instances but strive to address a certain degree of usability to accomplish a resemblance of consistency across kernel manager/client instances.

Resolves #455

(Note: the renaming of the gateway mapping kernel manager is not a compatibility issue because subclassing is currently not supported.)

[blink1073]

cc @declanvk

[codecov-commenter]

Codecov Report

Merging #483 (f4ba1ce) into master (a0135ca) will decrease coverage by 0.72%.
The diff coverage is 62.82%.

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
- Coverage   77.78%   77.06%   -0.73%     
==========================================
  Files         106      107       +1     
  Lines        9296     9487     +191     
  Branches     1003     1026      +23     
==========================================
+ Hits         7231     7311      +80     
- Misses       1705     1807     +102     
- Partials      360      369       +9     

Impacted Files	Coverage Δ
jupyter_server/gateway/managers.py	60.00% <55.00%> (-16.88%)	⬇️
jupyter_server/gateway/gateway_client.py	74.57% <74.57%> (ø)
jupyter_server/serverapp.py	67.23% <100.00%> (ø)
jupyter_server/tests/test_gateway.py	95.06% <100.00%> (ø)
jupyter_server/services/kernels/kernelmanager.py	80.36% <0.00%> (-0.73%)	⬇️

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[blink1073]

This one needs a rebase

[blink1073]

Thanks!