change migrations table to use snake case instead of camel case

Camel case is not supported by some database. Postgres for example
returns every column name in lower case. That does not lead to errors
but even more dangerous it jus silently fails when you try to access
`Migration::$executionTime`.

src/Migration/CreateMigrationTable.php

@@ -16,12 +16,12 @@ class CreateMigrationTable extends AbstractMigration
             reverted TIMESTAMP,
             status CHARACTER VARYING (16) NOT NULL DEFAULT 'done',
             statements TEXT,
-            executionTime DOUBLE PRECISION,
+            execution_time DOUBLE PRECISION,
             PRIMARY KEY (file)
         )");
         $this->exec("CREATE INDEX {$table}_executed_index ON {$table} (executed)");
         $this->exec("CREATE INDEX {$table}_status_index ON {$table} (status)");
-        $this->exec("CREATE INDEX {$table}_executionTime_index ON {$table} (executionTime)");
+        $this->exec("CREATE INDEX {$table}_execution_time_index ON {$table} (execution_time)");
     }
 
     /** @codeCoverageIgnore */

src/Migrations.php

@@ -283,7 +283,7 @@ class Migrations
         foreach ($migrations as $migration) {
             $this->progress->beforeMigration($migration);
             $this->statements = $migration->statements;
-            $start = microtime(true) - $migration->executionTime;
+            $start = microtime(true) - $migration->execution_time;
             try {
                 $this->db->beginTransaction();
                 $class = self::internalClass($migration->file) ??
@@ -367,7 +367,7 @@ class Migrations
 
         $migration->statements = $this->statements;
         $migration->status = $status;
-        $migration->executionTime = $executionTime;
+        $migration->execution_time = $executionTime;
 
         if (!$exists) {
             $this->db->prepare("INSERT INTO {$table}
@@ -378,7 +378,7 @@ class Migrations
                 $migration->executed->format('c'),
                 $migration->status,
                 json_encode($migration->statements),
-                $migration->executionTime
+                $migration->execution_time
             ]);
         } else {
             $this->db->prepare("UPDATE {$table} SET
@@ -389,7 +389,7 @@ class Migrations
                 $migration->reverted ? $migration->reverted->format('c') : null,
                 $migration->status,
                 json_encode($migration->statements),
-                $migration->executionTime,
+                $migration->execution_time,
                 $migration->file
             ]);
         }

src/Model/Migration.php

@@ -23,7 +23,7 @@ class Migration
     public $statements = [];
 
     /** @var double */
-    public $executionTime;
+    public $execution_time;
 
     public function __construct()
     {

tests/Migration/CreateMigrationTableTest.php

@@ -35,7 +35,7 @@ class CreateMigrationTableTest extends TestCase
             ->with(m::pattern('/create index.* on migrations\s*\(\s*status\s*\)/i'))
             ->once();
         $adapter->shouldHaveReceived('exec')
-            ->with(m::pattern('/create index.* on migrations\s*\(\s*executionTime\s*\)/i'))
+            ->with(m::pattern('/create index.* on migrations\s*\(\s*execution_time\s*\)/i'))
             ->once();
     }
 }