grpc / grpc-java / #20197
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coverage: 88.694% (+0.005%) from 88.689%
#20197

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xds: reuse GrpcXdsTransport and underlying gRPC channel to the same xDS server by ref-counting

This PR implements reusing the gRPC xDS transport (and underlying gRPC
channel) to the same xDS server by ref-counting, which is already
implemented in gRPC C++
([link](https://github.com/grpc/grpc/blob/5a3a5d531/src/core/xds/grpc/xds_transport_grpc.cc#L399-L414))
and gRPC Go
([link](https://github.com/grpc/grpc-go/blob/81c7924ec/internal/xds/clients/grpctransport/grpc_transport.go#L78-L120)).
This optimization is expected to reduce memory footprint of the xDS
management server and xDS enabled clients as channel establishment and
lifecycle management of the connection is expensive.

* Implemented a map to store `GrpcXdsTransport` instances keyed by the
`Bootstrapper.ServerInfo` and each `GrpcXdsTransport` has a ref count.
Note, the map cannot be simply keyed by the xDS server address as the
client could have different channel credentials to the same xDS server,
which should be counted as different transport instances.
* When `GrpcXdsTransportFactory.create()` is called, the existing
transport is reused if it already exists in the map and increment its
ref count, otherwise create a new transport, store it in the map, and
increment its ref count.
* When `GrpcXdsTransport.shutdown()` is called, its ref count is
decremented and the underlying gRPC channel is shut down when its ref
count reaches zero.
* Note this ref-counting of the `GrpcXdsTransport` is different and
orthogonal to the ref-counting of the xDS client keyed by the xDS server
target name to allow for xDS-based fallback per [gRFC
A71](https://github.com/grpc/proposal/blob/master/A71-xds-fallback.md).

Prod risk level: Low
* Reusing the underlying gRPC channel to the xDS server would not affect
the gRPC xDS (ADS/LRS) streams which would be multiplexed on the same
channel, however, this means new xDS (ADS/LRS) streams and RPCs may fail
due ... (continued)

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 -3.03% ../netty/src/main/java/io/grpc/netty/ClientTransportLifecycleManager.java
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Jobs
ID Job ID Ran Files Coverage
1 #20197.1 630
88.69
Source Files on build #20197