client/rpc: Change session identifier on reset, add unit tests

[kostko]

Previously the EnclaveRPC client did not change the session identifier on reset, resulting in unnecessary round-trips during a transport error. The EnclaveRPC client now changes the session identifier and the code now includes unit tests for the EnclaveRPC client (and demux).

[codecov]

Codecov Report

Merging #2872 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2872   +/-   ##
=======================================
  Coverage   67.70%   67.70%           
=======================================
  Files         344      344           
  Lines       33530    33530           
=======================================
+ Hits        22700    22703    +3     
- Misses       7915     7917    +2     
+ Partials     2915     2910    -5     

Impacted Files	Coverage Δ
go/consensus/tendermint/api/api.go	73.58% <0.00%> (-15.10%)	⬇️
go/runtime/committee/nodes.go	80.39% <0.00%> (-3.93%)	⬇️
go/storage/metrics.go	84.78% <0.00%> (-3.27%)	⬇️
go/consensus/tendermint/api/state.go	51.85% <0.00%> (-2.47%)	⬇️
go/consensus/tendermint/scheduler/scheduler.go	76.41% <0.00%> (-1.89%)	⬇️
go/worker/compute/txnscheduler/committee/node.go	64.50% <0.00%> (-1.86%)	⬇️
go/storage/database/database.go	79.09% <0.00%> (-1.82%)	⬇️
go/worker/keymanager/worker.go	58.50% <0.00%> (-1.50%)	⬇️
...o/consensus/tendermint/apps/scheduler/scheduler.go	67.63% <0.00%> (-1.32%)	⬇️
go/staking/api/grpc.go	52.61% <0.00%> (-0.99%)	⬇️
... and 19 more

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