Add TCP keepalive and TCP keepalive time configurations

## Summary

Related with #1489.

In Active Record Oracle enhanced adapter 5.2, an Oracle error occurs
when TCP keepalive time exceeds 600 seconds.

The following ORA-03113 error occurred every 10 minutes when I tried to
upgrade from Rails 5.1.6.1 (activerecord-oracle_enhanced-adapter 1.8.2)
to Rails 5.2.2 (activerecord-oracle_enhanced-adapter 5.2.5).

```console
stmt.c:243:in oci8lib_240.so: OCIError: ORA-03113: 通信チャネルで
end-of-fileが検出されました (ActiveRecord::StatementInvalid)
プロセスID: 24578
```

I think that TCP Keepalive parameters depend on an application purpose.
Therefore this PR allows users to change TCP keepalive parameters.

The default behavior has not changed from Active Record Oracle enhanced adapter 5.2
so that breaking changes don't occur.

### Other Information

TCP keepalive parameter names conform to ruby-oci8.
http://www.rubydoc.info/github/kubo/ruby-oci8/file/docs/hanging-after-inactivity.md

lib/active_record/connection_adapters/oracle_enhanced/oci_connection.rb

@@ -297,6 +297,8 @@ def create_time_with_default_timezone(value)
       # The OracleEnhancedOCIFactory factors out the code necessary to connect and
       # configure an Oracle/OCI connection.
       class OracleEnhancedOCIFactory #:nodoc:
+        DEFAULT_TCP_KEEPALIVE_TIME = 600
+
         def self.new_connection(config)
           # to_s needed if username, password or database is specified as number in database.yml file
           username = config[:username] && config[:username].to_s
@@ -326,11 +328,13 @@ def self.new_connection(config)
           else
             database
           end
-          OCI8.properties[:tcp_keepalive] = true
+
+          OCI8.properties[:tcp_keepalive] = config[:tcp_keepalive] == false ? false : true
           begin
-            OCI8.properties[:tcp_keepalive_time] = 600
+            OCI8.properties[:tcp_keepalive_time] = config[:tcp_keepalive_time] || DEFAULT_TCP_KEEPALIVE_TIME
           rescue NotImplementedError
           end
+
           conn = OCI8.new username, password, connection_string, privilege
           conn.autocommit = true
           conn.non_blocking = true if async

lib/active_record/connection_adapters/oracle_enhanced_adapter.rb

@@ -115,6 +115,8 @@ module ConnectionAdapters #:nodoc:
     # * <tt>:time_zone</tt> - database session time zone
     #   (it is recommended to set it using ENV['TZ'] which will be then also used for database session time zone)
     # * <tt>:schema</tt> - database schema which holds schema objects.
+    # * <tt>:tcp_keepalive</tt> - TCP keepalive is enabled for OCI client, defaults to true
+    # * <tt>:tcp_keepalive_time</tt> - TCP keepalive time for OCI client, defaults to 600
     #
     # Optionals NLS parameters:
     #

spec/active_record/connection_adapters/oracle_enhanced/connection_spec.rb

@@ -135,6 +135,34 @@
     end
   end
 
+  if defined?(OCI8)
+    describe "with TCP keepalive parameters" do
+      it "should use database default `tcp_keepalive` value true by default" do
+        ActiveRecord::ConnectionAdapters::OracleEnhanced::Connection.create(CONNECTION_PARAMS)
+
+        expect(OCI8.properties[:tcp_keepalive]).to be true
+      end
+
+      it "should use modified `tcp_keepalive` value false" do
+        ActiveRecord::ConnectionAdapters::OracleEnhanced::Connection.create(CONNECTION_PARAMS.dup.merge(tcp_keepalive: false))
+
+        expect(OCI8.properties[:tcp_keepalive]).to be false
+      end
+
+      it "should use database default `tcp_keepalive_time` value 600 by default" do
+        ActiveRecord::ConnectionAdapters::OracleEnhanced::Connection.create(CONNECTION_PARAMS)
+
+        expect(OCI8.properties[:tcp_keepalive_time]).to eq(600)
+      end
+
+      it "should use modified `tcp_keepalive_time` value 3000" do
+        ActiveRecord::ConnectionAdapters::OracleEnhanced::Connection.create(CONNECTION_PARAMS.dup.merge(tcp_keepalive_time: 3000))
+
+        expect(OCI8.properties[:tcp_keepalive_time]).to eq(3000)
+      end
+    end
+  end
+
   describe "with non-string parameters" do
     before(:all) do
       params = CONNECTION_PARAMS.dup