Add mutex on datastore I/O operations

To make sure that there is no concurrent operation on the datastore file.

Fixes #1132

composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "f84918821b0dceb0cd569875c0418bb8",
+    "content-hash": "932b191006135ff8be495aa0b4ba7e09",
     "packages": [
         {
             "name": "arthurhoaro/web-thumbnailer",
@@ -344,6 +344,91 @@
             },
             "time": "2016-11-07T19:29:14+00:00"
         },
+        {
+            "name": "malkusch/lock",
+            "version": "v2.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/php-lock/lock.git",
+                "reference": "093f389ec2f38fc8686d2f70e23378182fce7714"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/php-lock/lock/zipball/093f389ec2f38fc8686d2f70e23378182fce7714",
+                "reference": "093f389ec2f38fc8686d2f70e23378182fce7714",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=7.1",
+                "psr/log": "^1"
+            },
+            "require-dev": {
+                "eloquent/liberator": "^2.0",
+                "ext-memcached": "*",
+                "ext-pcntl": "*",
+                "ext-pdo_mysql": "*",
+                "ext-pdo_sqlite": "*",
+                "ext-redis": "*",
+                "ext-sysvsem": "*",
+                "johnkary/phpunit-speedtrap": "^3.0",
+                "kriswallsmith/spork": "^0.3",
+                "mikey179/vfsstream": "^1.6",
+                "php-mock/php-mock-phpunit": "^2.1",
+                "phpunit/phpunit": "^7.4",
+                "predis/predis": "^1.1",
+                "squizlabs/php_codesniffer": "^3.3"
+            },
+            "suggest": {
+                "ext-pnctl": "Enables locking with flock without busy waiting in CLI scripts.",
+                "ext-redis": "To use this library with the PHP Redis extension.",
+                "ext-sysvsem": "Enables locking using semaphores.",
+                "predis/predis": "To use this library with predis."
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "malkusch\\lock\\": "classes/"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "WTFPL"
+            ],
+            "authors": [
+                {
+                    "name": "Markus Malkusch",
+                    "email": "markus@malkusch.de",
+                    "homepage": "http://markus.malkusch.de",
+                    "role": "Developer"
+                },
+                {
+                    "name": "Willem Stuursma-Ruwen",
+                    "email": "willem@stuursma.name",
+                    "role": "Developer"
+                }
+            ],
+            "description": "Mutex library for exclusive code execution.",
+            "homepage": "https://github.com/malkusch/lock",
+            "keywords": [
+                "advisory-locks",
+                "cas",
+                "flock",
+                "lock",
+                "locking",
+                "memcache",
+                "mutex",
+                "mysql",
+                "postgresql",
+                "redis",
+                "redlock",
+                "semaphore"
+            ],
+            "support": {
+                "issues": "https://github.com/php-lock/lock/issues",
+                "source": "https://github.com/php-lock/lock/tree/v2.1"
+            },
+            "time": "2018-12-12T19:53:29+00:00"
+        },
         {
             "name": "nikic/fast-route",
             "version": "v1.3.0",