Protobuf Release v3.16.3

Java

• Refactoring java full runtime to reuse sub-message builders and prepare to
  migrate parsing logic from parse constructor to builder.
• Move proto wireformat parsing functionality from the private "parsing
  constructor" to the Builder class.
• Change the Lite runtime to prefer merging from the wireformat into mutable
  messages rather than building up a new immutable object before merging. This
  way results in fewer allocations and copy operations.
• Make message-type extensions merge from wire-format instead of building up
  instances and merging afterwards. This has much better performance.
• Fix TextFormat parser to build up recurring (but supposedly not repeated)
  sub-messages directly from text rather than building a new sub-message and
  merging the fully formed message into the existing field.
• This release addresses a Security Advisory for Java users