Making getters dictionary static

[gmanvel]

This PR introduces 2 small improvements.

First is making gettersDictionary static, so that it is reused across AvroConvert.Serialize calls, e.g. currently this will cause gettersDictionary to be populated on each AvroConvert.Serialize call

var users = new List<User>();
AvroConvert.Serialize(users);
AvroConvert.Serialize(users);

Making it static will ensure it is populated only once. Given that GenerateGetValue is an expensive operation eliminating its execution would improve performance.

Second suggested improvement is gettersDictionary population if race conditions occur. Normally if 2 or more threads try to execute GenerateGetValue concurrently to store the value in dictionary, all of them will execute but only single return value will be used. Since GenerateGetValue is an expensive operation, using Lazy as its value will ensure that even if multiple threads try to create value, they will create Lazy instance which is much cheaper. Later on when value is accessed, Lazy will ensure only 1 GenerateGetValue is executed avoiding to run multiple expensive operations altogether. See this blog post for more details about this improvement.

[AdrianStrugala]

Thank you for another excellent idea!