Fix Relationship.instances cache.

This PR aims to fix several issues with Relationship cache:

1) It's not threadsafe, so I propose to use a TLS variable for this.
2) Memory obtained by cache remains non-freed before the next run of `serialize`. I think it should be freed immediately.
3) Memory should be freed in `ensure` block to prevent memory bloating in case of exception.

*There are only two hard things in Computer Science: cache invalidation and naming things.*

lib/axlsx/package.rb

@@ -100,11 +100,13 @@ def workbook=(workbook) DataTypeValidator.validate :Package_workbook, Workbook,
     #   File.open('example_streamed.xlsx', 'w') { |f| f.write(s.read) }
     def serialize(output, confirm_valid=false)
       return false unless !confirm_valid || self.validate.empty?
-      Relationship.clear_cached_instances
+      Relationship.initialize_cached_instances
       Zip::OutputStream.open(output) do |zip|
         write_parts(zip)
       end
       true
+    ensure
+      Relationship.clear_cached_instances
     end
 
 
@@ -113,11 +115,13 @@ def serialize(output, confirm_valid=false)
     # @return [StringIO|Boolean] False if confirm_valid and validation errors exist. rewound string IO if not.
     def to_stream(confirm_valid=false)
       return false unless !confirm_valid || self.validate.empty?
-      Relationship.clear_cached_instances
+      Relationship.initialize_cached_instances
       zip = write_parts(Zip::OutputStream.new(StringIO.new, true))
       stream = zip.close_buffer
       stream.rewind
       stream
+    ensure
+      Relationship.clear_cached_instances
     end
 
     # Encrypt the package into a CFB using the password provided

lib/axlsx/rels/relationship.rb

@@ -8,20 +8,28 @@ class << self
       # Keeps track of all instances of this class.
       # @return [Array]
       def instances
-        @instances ||= []
+        Thread.current[:axlsx_relationship_cached_instances] ||= []
       end
-      
-      # Clear cached instances.
+
+      # Initialize cached instances.
       # 
       # This should be called before serializing a package (see {Package#serialize} and
       # {Package#to_stream}) to make sure that serialization is idempotent (i.e. 
       # Relationship instances are generated with the same IDs everytime the package
       # is serialized).
+      def initialize_cached_instances
+        Thread.current[:axlsx_relationship_cached_instances] = []
+      end
+
+      # Clear cached instances.
+      # 
+      # This should be called after serializing a package (see {Package#serialize} and
+      # {Package#to_stream}) to free the memory allocated for cache.
       # 
       # Also, calling this avoids memory leaks (cached instances lingering around 
       # forever). 
       def clear_cached_instances
-        @instances = []
+        Thread.current[:axlsx_relationship_cached_instances] = nil
       end
 
       # Generate and return a unique id (eg. `rId123`) Used for setting {#Id}. 
@@ -30,7 +38,7 @@ def clear_cached_instances
       # {clear_cached_instances} will automatically reset the generated ids, too.
       # @return [String]
       def next_free_id
-        "rId#{@instances.size + 1}"
+        "rId#{instances.size + 1}"
       end
     end