This change introduces garbage collection for the light client. Historical
chain data is deleted periodically. If you want to disable the GC, use
the --light.nopruning flag.
# Conflicts:
#	cmd/geth/main.go
#	cmd/utils/flags.go
#	core/blockchain.go
#	core/chain_indexer.go
#	core/chain_indexer_test.go
#	core/chain_makers.go
#	core/dao_test.go
#	core/genesis.go
#	core/rawdb/accessors_chain_test.go
#	core/rawdb/accessors_indexes_test.go
#	core/rawdb/freezer.go
#	core/state/statedb_test.go
#	eth/downloader/downloader.go
#	eth/downloader/downloader_test.go
#	eth/downloader/testchain_test.go
#	eth/gen_config.go
#	internal/ethapi/api.go
#	internal/ethapi/backend.go
#	les/api_backend.go
#	les/client.go
#	les/odr_requests.go
#	les/odr_test.go
#	les/request_test.go
#	les/server.go
#	les/sync_test.go
#	les/test_helper.go
#	les/ulc_test.go
#	light/lightchain.go
#	light/odr.go
#	light/odr_util.go
#	light/postprocess.go
#	trie/database.go
#	trie/iterator_test.go
#	trie/trie_test.go