Improve Structure (subclass) construction performance.

[brettwooldridge]

Fixes #1625 issue by caching the field list (fieldList) and validation state (validationMap) in addition to the existing layoutInfo and fieldOrder caches.

Caching this additional state provides substantial performance improvements.

[matthiasblaesing]

@brettwooldridge thank you. Two requests:

• Do you have performance numbers for this?
• Please add an entry to CHANGES.md (refer to the existing entries for the target format).

Edit: Please also squash the changes when updating the changes file.

[brettwooldridge]

@matthiasblaesing I have refactored the change using ReentrantReadWriteLock to substantially decrease contention as this is a 99.9% read/0.1% write type access pattern. Additionally, I updated the CHANGES.md as requested. All tests are passing.

[dbwiddis]

Thanks for the extra effort here! I agree Structures are one of the biggest opportunities for performance improvements; the improvements in releasing the backing Memory in a previous release were pretty significant (and I still think there's room to improve with a "cache" of the common 4- or 8- byte allocations) it's good to see unnecessary redundant calculations elimitated as well.

[matthiasblaesing]

Thank you again and sorry for the delay in merging. Lets get this in.