Don't use Concurrent::{Hash|Array} objects in Fields

Since the fields are (through the thread-local BufferStack) only ever
accessed from a single Thread, there is no need to accept the additional
locking overhead of the Concurrent raw values.

We can just use simple Hashes and Arrays here for higher performance.

lib/rackstash/fields/abstract_collection.rb

@@ -115,7 +115,7 @@ module Rackstash
         when Rackstash::Fields::Hash, Rackstash::Fields::Array
           return wrap ? value : value.raw
         when ::Hash
-          hash = Concurrent::Hash.new
+          hash = {}
           value.each_pair do |k, v|
             hash[utf8_encode(k)] = normalize(v, scope: scope)
           end
@@ -126,10 +126,7 @@ module Rackstash
           end
           return hash
         when ::Array, ::Set, ::Enumerator
-          array = Concurrent::Array.new
-          value.each do |e|
-            array << normalize(e, scope: scope)
-          end
+          array = value.map { |e| normalize(e, scope: scope) }
           if wrap
             array = Rackstash::Fields::Array.new.tap do |array_field|
               array_field.raw = array

lib/rackstash/fields/array.rb

@@ -11,7 +11,7 @@ module Rackstash
   module Fields
     class Array < AbstractCollection
       def initialize
-        @raw = Concurrent::Array.new
+        @raw = []
       end
 
       # @!method +(array)
@@ -93,7 +93,7 @@ module Rackstash
       #     start index is out of range
       def [](index, length = nil)
         result = length.nil? ? @raw[index] : @raw[index, length]
-        result = new(result) if ::Concurrent::Array === result
+        result = new(result) if ::Array === result
         result
       end
       alias slice []
@@ -167,7 +167,7 @@ module Rackstash
 
       # @return [::Array] deep-transforms the array into a plain Ruby Array
       def as_json(*)
-        @raw.to_a.map! { |value|
+        @raw.map { |value|
           value.is_a?(AbstractCollection) ? value.as_json : value
         }
       end

lib/rackstash/fields/hash.rb

@@ -17,7 +17,7 @@ module Rackstash
       # @param forbidden_keys [Set<String>,::Array<String>] a list of strings
       #   which are not allowed to be used as keys in this hash
       def initialize(forbidden_keys: EMPTY_SET)
-        @raw = Concurrent::Hash.new
+        @raw = {}
 
         unless forbidden_keys.is_a?(Set) &&
                forbidden_keys.frozen? &&
@@ -62,7 +62,7 @@ module Rackstash
 
       # @return [::Hash] deep-transforms the hash into a plain Ruby Hash
       def as_json(*)
-        hash = @raw.to_h
+        hash = @raw.dup
         hash.each_pair do |key, value|
           hash[key] = value.as_json if value.is_a?(AbstractCollection)
         end

lib/rackstash/fields/tags.rb

@@ -13,7 +13,7 @@ module Rackstash
   module Fields
     class Tags < AbstractCollection
       def initialize
-        @raw = Concurrent::Hash.new
+        @raw = {}
       end
 
       def <<(tag)

spec/rackstash/fields/abstract_collection_spec.rb

@@ -198,9 +198,9 @@ describe Rackstash::Fields::AbstractCollection do
         expect(normalize(hash, wrap: false).keys).to all be_frozen
       end
 
-      it 'returns a Concurrent::Hash with wrap: false' do
+      it 'returns a ::Hash with wrap: false' do
         hash = { 'one' => 1 }
-        expect(normalize(hash, wrap: false)).to be_an_instance_of(Concurrent::Hash)
+        expect(normalize(hash, wrap: false)).to be_an_instance_of(::Hash)
       end
 
       it 'normalizes all values' do
@@ -267,9 +267,9 @@ describe Rackstash::Fields::AbstractCollection do
         expect(normalize(array, wrap: false)).to all be_frozen
       end
 
-      it 'returns a Concurrent::Array with wrap: false' do
+      it 'returns an ::Array with wrap: false' do
         array = [1, :two, 'three']
-        expect(normalize(array, wrap: false)).to be_an_instance_of(Concurrent::Array)
+        expect(normalize(array, wrap: false)).to be_an_instance_of(::Array)
       end
 
       it 'normalizes all values' do