Move ThreadLocal implementation into .cc

[yuslepukhin]

No description provided.

[yuslepukhin]

Failures does not seem to be related to this PR

[facebook-github-bot]

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[siying]

Looks good to me. I'm curious what's the reason for doing that? Did some code analyze tool complained?

[yuslepukhin]

@siying The reason for this a memory isolation project that I am doing. For some global pieces it is easier to subst a .cc file and not touch the header until I come up with a better way of doing this. This and another PR are the last in the series. No more expected.
In this project I have to allocate different instances of databases in different pools and thus thread-local storage suddenly acquires one more dimension. I need to track the actual memory pool where the thread-local was allocated from. Same thing for global threadpools. Each task submitted must execute within the memory pool context of the instance it submitted it.