Build out backoff as a library

BUILD

Makefile

binding.gyp

build.yaml

config.m4

gRPC.podspec

grpc.gemspec

package.json

package.xml

src/core/support/backoff.c

src/core/support/backoff.h

src/python/grpcio/grpc_core_dependencies.py

test/core/support/backoff_test.c

tools/doxygen/Doxyfile.core.internal

tools/run_tests/sources_and_headers.json

tools/run_tests/tests.json

vsprojects/buildtests_c.sln

vsprojects/vcxproj/gpr/gpr.vcxproj

vsprojects/vcxproj/gpr/gpr.vcxproj.filters

...ts/vcxproj/test/gpr_backoff_test/gpr_backoff_test.vcxproj