Update code to work with recent abseil changes

[kkufieta]

This patch fixes compilation issues caused by the most recent changes to abseil-cpp: https://github.com/abseil/abseil-cpp/releases:

The legacy spellings of the thread annotation macros/functions (e.g. GUARDED_BY()) have been removed by default in favor of the ABSL_ prefixed versions (e.g. ABSL_GUARDED_BY()) due to clashes with other libraries. The compatibility macro ABSL_LEGACY_THREAD_ANNOTATIONS can be defined on the compile command-line to temporarily restore these spellings, but this compatibility macro will be removed in the future.

[kkufieta]

Gentle ping - I would love to get these changes merged so we can stop using the ABSL_LEGACY_THREAD_ANNOTATIONS compatibility macro before it is going to be removed in the future.