More generic configuration system is introduced in order to i) unify the
way how modules access the configurations instead of using low-level
get/setenv functions and ii) enable the customization for where configuration
is stored. This could be extended to support flag, file, etc.
Default configuration system uses environment variables as before so
basically this is expected to work just as it did. This behavior can
change by redefining GPR_GLOBAL_CONFIG_DEFINE_*type* macros.
* Migrated configuration
GRPC_CLIENT_CHANNEL_BACKUP_POLL_INTERVAL_MS
GRPC_EXPERIMENTAL_DISABLE_FLOW_CONTROL
GRPC_ABORT_ON_LEAKS
GRPC_NOT_USE_SYSTEM_SSL_ROOTS
Esun Kim