Bump protobuf from 2.6.1 to 3.15.0

[dependabot]

Bumps protobuf from 2.6.1 to 3.15.0.

Release notes

Sourced from protobuf's releases.

Protocol Buffers v3.15.0

Protocol Compiler

• Optional fields for proto3 are enabled by default, and no longer require the --experimental_allow_proto3_optional flag.

C++

• MessageDifferencer: fixed bug when using custom ignore with multiple unknown fields
• Use init_seg in MSVC to push initialization to an earlier phase.
• Runtime no longer triggers -Wsign-compare warnings.
• Fixed -Wtautological-constant-out-of-range-compare warning.
• DynamicCastToGenerated works for nullptr input for even if RTTI is disabled
• Arena is refactored and optimized.
• Clarified/specified that the exact value of Arena::SpaceAllocated() is an implementation detail users must not rely on. It should not be used in unit tests.
• Change the signature of Any::PackFrom() to return false on error.
• Add fast reflection getter API for strings.
• Constant initialize the global message instances
• Avoid potential for missed wakeup in UnknownFieldSet
• Now Proto3 Oneof fields have "has" methods for checking their presence in C++.
• Bugfix for NVCC
• Return early in _InternalSerialize for empty maps.
• Adding functionality for outputting map key values in proto path logging output (does not affect comparison logic) and stop printing 'value' in the path. The modified print functionality is in the MessageDifferencer::StreamReporter.
• Fixed protocolbuffers/protobuf#8129
• Ensure that null char symbol, package and file names do not result in a crash.
• Constant initialize the global message instances
• Pretty print 'max' instead of numeric values in reserved ranges.
• Removed remaining instances of std::is_pod, which is deprecated in C++20.
• Changes to reduce code size for unknown field handling by making uncommon cases out of line.
• Fix std::is_pod deprecated in C++20 (#7180)
• Fix some -Wunused-parameter warnings (#8053)
• Fix detecting file as directory on zOS issue #8051 (#8052)
• Don't include sys/param.h for _BYTE_ORDER (#8106)
• remove CMAKE_THREAD_LIBS_INIT from pkgconfig CFLAGS (#8154)
• Fix TextFormatMapTest.DynamicMessage issue#5136 (#8159)
• Fix for compiler warning issue#8145 (#8160)
• fix: support deprecated enums for GCC < 6 (#8164)
• Fix some warning when compiling with Visual Studio 2019 on x64 target (#8125)

Python

• Provided an override for the reverse() method that will reverse the internal collection directly instead of using the other methods of the BaseContainer.
• MessageFactory.CreateProtoype can be overridden to customize class creation.

... (truncated)

Commits

• ae50d9b Update protobuf version
• 8260126 Update protobuf version
• c741c46 Resovled issue in the .pb.cc files
• eef2764 Resolved an issue where NO_DESTROY and CONSTINIT were in incorrect order
• 0040102 Updated collect_all_artifacts.sh for Ubuntu Xenial
• 26cb6a7 Delete root-owned files in Kokoro builds
• 1e924ef Update port_def.inc
• 9a80cf1 Update coded_stream.h
• a97c4f4 Merge pull request #8276 from haberman/php-warning
• 44cd75d Merge pull request #8282 from haberman/changelog
• Additional commits viewable in compare view

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[dependabot]

Superseded by #35.