services/horizon: Add --skip-migrations-check flag (stellar#3557)

This commit adds `--skip-migrations-check` flag that, when set, will skip
checking if there are any migrations required to start Horizon.

When checking if there are any migrations required Horizon calls
`migrate.PlanMigration` which sends `CREATE TABLE gorp_migrations`. This query
is forbidden when using read only database (like replica).

services/horizon/internal/config.go

@@ -74,6 +74,9 @@ type Config struct {
 	// ApplyMigrations will apply pending migrations to the horizon database
 	// before starting the horizon service
 	ApplyMigrations bool
+	// SkipMigrationsCheck will skip checking if there are any migrations
+	// required
+	SkipMigrationsCheck bool
 	// CheckpointFrequency establishes how many ledgers exist between checkpoints
 	CheckpointFrequency uint32
 	// BehindCloudflare determines if Horizon instance is behind Cloudflare. In

services/horizon/internal/flags.go

@@ -386,6 +386,14 @@ func Flags() (*Config, support.ConfigOptions) {
 			Required:    false,
 			Usage:       "applies pending migrations before starting horizon",
 		},
+		&support.ConfigOption{
+			Name:        "skip-migrations-check",
+			ConfigKey:   &config.SkipMigrationsCheck,
+			OptType:     types.Bool,
+			FlagDefault: false,
+			Required:    false,
+			Usage:       "skips checking if there are migrations required",
+		},
 		&support.ConfigOption{
 			Name:        "checkpoint-frequency",
 			ConfigKey:   &config.CheckpointFrequency,
@@ -443,7 +451,9 @@ func ApplyFlags(config *Config, flags support.ConfigOptions) {
 	}
 
 	// Migrations should be checked as early as possible
-	checkMigrations(*config)
+	if !config.SkipMigrationsCheck {
+		checkMigrations(*config)
+	}
 
 	// Validate options that should be provided together
 	validateBothOrNeither("tls-cert", "tls-key")