Backport #6759 and disable extension code gen for C#

Update CHANGES.txt with changes since 3.10.0-RC1

* Initialize well known type values

Also fixes #6667

* Revert unwanted change

The new function name also better reflects the semantics of the
function.  Like timegm(), this function always converts to/from
UTC, not local time.

Pull the ObjC fixes from #6679 to the 3.10.x branch

The OSReadLittleInt64 function as defined by Apple reduces down to:

`return *(volatile uint64_t *)((uintptr_t)base + byteOffset);`

which means we are type-punning using a cast. On ARMv7 and other aligned architectures this can cause crashes. 
Minimal example: https://gist.github.com/dmaclach/b10b0a71ae614d304c067cb9bd264336

Fixes #6679 

The code in question hasn't change in a long time so the cause of
https://github.com/firebase/firebase-ios-sdk/issues/3851 still appears to be
an Xcode 11 clang change/bug; but this does appear to be slightly better
code for the work being done.

Cleanup along the way for #6679 

Update protobuf version to 3.10.0 stable

Fix update_version.py, so that PROTOBUF_VERSION is always a 9 digit number

Update protobuf version for Java and PHP

Cherry Pick (#6614)

* Sync upb

https://github.com/protocolbuffers/upb/pull/209

* Update upb

https://github.com/protocolbuffers/upb/pull/210

Java: Add Automatic-Module-Name entries to the Manifest

This commit removes the question template. It also updates the bug report and feature request templates to direct questions to the mailing list.

* When length is zero, substr returns null instead of emptry string, which breaks the invariable for message.
* Tested in https://github.com/protocolbuffers/protobuf/pull/6560

Update version number

* For missing message value, map should create a default message
instance in order to keep its invariable.
* On 32-bit platform, int64 map key should be string

We've notice significant issues downloading from zlib.net. Since there
are already other archives coming from GitHub, this shouldn't negatively
affect reliability.

Optimization for layout_init()

* Add missing files under benchmarks/ to the dist

* fix typo

* Sync upb

* https://github.com/protocolbuffers/upb/pull/208

* Fix php c extension compiling

Don't use macros defined by upb

* Update ruby conformance failure list

Enums use varint, so if the value is negative it should end up being longer.

This was caught my new conformance test cases.