* Add document for php development

* Fix comments

Update Java Readme to 3.9.2

3.9.2 is on the Githib releases page.

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 

Fix incorrect Proto2 C# doc comments

* Cherry pick changes on update_version.py #6643

* Update version to 3.10.0-rc-0

* Do not add rc0 to php packages.xml

Simplify template exporting macros

* Down integrate to GitHub

* Remove unintentional double assign

* Change ByteCountInt64 to int64_t

* Revert changes in binary_conformance_test.cc

* Revert unnecessary changes

Remove duplicate entry in third party list

* Implement binary conformance test for oneof fields

* Update failure list

* Update failure list for php c extension

On the linked Wiki page, it can be seen that the creator is `opticron`, whose Github repository is also linked, and contains a more up-to-date implementation.

Linakge Monitor fix to handle a new version of protobuf-bom not available in Maven Central

* Sync upb

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

* Update upb

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

- use final instead of obvious types for local variables
- add missing `void` return type
- make the comments doccomments (///)

This adds links under the Clojure header to

https://protojure.github.ioSigned-off-by: Greg Haskins <gregory.haskins@gmail.com>

* Add binary conformance tests for map fields

* Update failure list

* Fix php conformance test

* Fix php conformance test

In 32-bit platform, int64 should be string. However, map iterator returns string key as integer.

* Add more test cases for map

* Update failure list

* introduce GOOGLE_PROTOBUF_SUPPORT_SYSTEM_MEMORY

* allow building net45 target on unix systems too

* add Span<> support on net45 as well

Document proto2 feature implementations in Google.Protobuf

C# Proto2 feature : Finale

Reworded info about message initialization

Addressed review comments

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