fn: lb and pure-runner with non-blocking agent (#989)

* fn: lb and pure-runner with non-blocking agent

*) Removed pure-runner capacity tracking code. This did
not play well with internal agent resource tracker.
*) In LB and runner gRPC comm, removed ACK. Now,
upon TryCall, pure-runner quickly proceeds to call
Submit. This is good since at this stage pure-runner
already has all relevant data to initiate the call.
*) Unless pure-runner emits a NACK, LB immediately
streams http body to runners.
*) For retriable requests added a CachedReader for
http.Request Body.
*) Idempotenty/retry is similar to previous code.
After initial success in Engament, after attempting
a TryCall, unless we receive NACK, we cannot retry
that call.
*) ch and naive places now wraps each TryExec with
a cancellable context to clean up gRPC contexts
quicker.

* fn: err for simpler one-time read GetBody approach

This allows for a more flexible approach since we let
users to define GetBody() to allow repetitive http body
read. In default LB case, LB executes a one-time io.ReadAll
and sets of GetBody, which is detected by RunnerCall.RequestBody().

* fn: additional check for non-nil req.body

* fn: attempt to override IO errors with ctx for TryExec

* fn: system-tests log dest

* fn: LB: EOF send handling

* fn: logging for partial IO

* fn: use buffer pool for IO storage in lb agent

* fn: pure runner should use chunks for data msgs

* fn: required config validations and pass APIErrors

* fn: additional tests and gRPC proto simplification

*) remove ACK/NACK messages as Finish message type works
OK for this purpose.
*) return resp in api tests for check for status code
*) empty body json test in api tests for lb & pure-runner

* fn: buffer adjustments

*) setRequestBody result handling correction
*) switch to bytes.Reader for read-only safety
*) io.EOF can be returned for non-nil Body in request.

* fn: clarify detection of 503 / Server Too Busy

system_test.sh

@@ -39,6 +39,12 @@ esac
 # avoid port conflicts with api_test.sh which are run in parallel
 export FN_API_URL="http://localhost:8085"
 export FN_DS_DB_PING_MAX_RETRIES=60
+
+# pure runner and LB agent required settings below
+export FN_MAX_REQUEST_SIZE=6291456
+export FN_MAX_RESPONSE_SIZE=6291456
+export FN_ENABLE_NB_RESOURCE_TRACKER=1
+
 cd test/fn-system-tests && FN_DB_URL=${FN_DB_URL} FN_API_URL=${FN_API_URL} go test -v -parallel ${2:-1} ./...; cd ../../
 
 remove_system_containers