Merge with changes that implement config file PR #15548.
Field *EnsApi string* in swarm/api.Config is replaced with
*EnsAPIs []string*.
A new field *EnsDisabled bool* is added to swarm/api.Config for
easy way to disable ENS resolving with config file.
Signature of function swarm.NewSwarm is changed and simplified.
This commit adds a TOML configuration option to swarm. It reuses
the TOML configuration structure used in geth with swarm
customized items.
The commit:
* Adds a "dumpconfig" command to the swarm executable which
allows printing the (default) configuration to stdout, which
then can be redirected to a file in order to customize it.
* Adds a "--config <file>" option to the swarm executable which will
allow to load a configuration file in TOML format from the
specified location in order to initialize the Swarm node The
override priorities are like follows: environment variables
override command line arguments override config file override
default config.
