Fix or inhibit various compiler warnings while building on clang.

[TimothyBanks]

Change Description

There were a few compiler warnings occurring while building eos on clang:

• There were a few warning pragmas within the code. I removed those and created JIRA issues to address them.
• A few 3rdParty libraries not maintained by block.one produce various compiler warnings when built on clang. I decided to inhibit all compiler warnings in those projects.
• The build produced quite a few ld: warning: direct access in weak symbol. I eliminated the majority of them by changing the default symbol visibility to hidden. There are still a couple that are being generated when linking against libpqxx because it seems that library isn't using hidden by default.

Change Type

Select ONE:

• Documentation

• Stability bug fix

• Other

• Other - special case

Testing Changes

Select ANY that apply:

• New Tests

• Existing Tests

• Test Framework

• CI System

• Other

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions

[linhuang-blockone]

Would it be helpful to list the JIRAs here?

[TimothyBanks]

I was told not to list JIRAs in the PR. But there is only one JIRA issue for resolving the compilation warnings on MacOS.