runtime: Make dispatcher core async

[kostko]

Fixes #3972

Note that this only affects the core and allows for parallel query
processing which still happens in blocking threads. It does open the
way for fully async query dispatch.

[codecov]

Codecov Report

Merging #4273 (e317ce1) into master (69ffe1b) will decrease coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4273      +/-   ##
==========================================
- Coverage   69.14%   68.79%   -0.36%     
==========================================
  Files         410      410              
  Lines       47130    47130              
==========================================
- Hits        32587    32421     -166     
- Misses      10522    10703     +181     
+ Partials     4021     4006      -15     

Impacted Files	Coverage Δ
go/ias/http/http.go	20.33% <0.00%> (-44.07%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.51% <0.00%> (-20.69%)	⬇️
go/keymanager/api/policy_sgx.go	33.33% <0.00%> (-13.34%)	⬇️
go/runtime/host/sandbox/sandbox.go	67.76% <0.00%> (-13.19%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/oasis-node/cmd/ias/auth.go	68.96% <0.00%> (-6.90%)	⬇️
go/common/sgx/aesm/aesm.go	63.63% <0.00%> (-6.37%)	⬇️
...nsensus/tendermint/apps/beacon/state/state_pvss.go	75.00% <0.00%> (-6.25%)	⬇️
go/runtime/host/protocol/connection.go	66.02% <0.00%> (-4.64%)	⬇️
go/runtime/host/helpers.go	71.42% <0.00%> (-4.09%)	⬇️
... and 41 more

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