fix all bits broken by viper API changes (#5982)

github.com/spf13/viper's recent releases introduced a semantic
change in some public API such as viper.IsSet(), which have
broken some of our flags checks. Instead of checking whether
users have changed a flag's default value we should rely on such
defaults and adjust runtime behaviour accordingly. In order to do
so, it's important that we pick sane defaults for all our flags.

The --pruning flag and configuration option now allow for a
fake custom strategy. When users elect custom, then the
pruning-{keep,snapshot}-every options are interpreted and
parsed; else they're ignored.
Zero is pruning-{keep,snapshot}-every default value. When
users choose to set a custom pruning strategy they are
signalling that they want more fine-grainted control, therefore
it's legitimate to expect them to know what they are doing and
enter valid values for both options.

Ref #5964

server/config/config.go

@@ -35,7 +35,9 @@ type BaseConfig struct {
 	// InterBlockCache enables inter-block caching.
 	InterBlockCache bool `mapstructure:"inter-block-cache"`
 
-	Pruning string `mapstructure:"pruning"`
+	Pruning              string `mapstructure:"pruning"`
+	PruningKeepEvery     string `mapstructure:"pruning-keep-every"`
+	PruningSnapshotEvery string `mapstructure:"pruning-snapshot-every"`
 }
 
 // Config defines the server's top level configuration
@@ -74,9 +76,11 @@ func (c *Config) GetMinGasPrices() sdk.DecCoins {
 func DefaultConfig() *Config {
 	return &Config{
 		BaseConfig{
-			MinGasPrices:    defaultMinGasPrices,
-			InterBlockCache: true,
-			Pruning:         store.PruningStrategySyncable,
+			MinGasPrices:         defaultMinGasPrices,
+			InterBlockCache:      true,
+			Pruning:              store.PruningStrategySyncable,
+			PruningKeepEvery:     "0",
+			PruningSnapshotEvery: "0",
 		},
 	}
 }

server/config/toml.go

@@ -34,11 +34,16 @@ halt-time = {{ .BaseConfig.HaltTime }}
 # InterBlockCache enables inter-block caching.
 inter-block-cache = {{ .BaseConfig.InterBlockCache }}
 
-# Pruning sets the pruning strategy: syncable, nothing, everything
+# Pruning sets the pruning strategy: syncable, nothing, everything, custom
 # syncable: only those states not needed for state syncing will be deleted (keeps last 100 + every 10000th)
 # nothing: all historic states will be saved, nothing will be deleted (i.e. archiving node)
 # everything: all saved states will be deleted, storing only the current state
+# custom: allows fine-grained control through the pruning-keep-every and pruning-snapshot-every options.
 pruning = "{{ .BaseConfig.Pruning }}"
+
+# These are applied if and only if the pruning strategy is custom.
+pruning-keep-every = "{{ .BaseConfig.PruningKeepEvery }}"
+pruning-snapshot-every = "{{ .BaseConfig.PruningSnapshotEvery }}"
 `
 
 var configTemplate *template.Template