[C][Client]Fix coredump in multi-thread environment

[ityuhui]

@wing328 @zhemant @michelealbano

Described in libcurl official document https://curl.haxx.se/libcurl/c/libcurl.html, the functions curl_global_init and curl_global_cleanup cannot be called in multiple thread environment, or it will cause a coredump.

Following by the guide, this PR removes the 2 function calls from apiClient_create* and apiClient_free, and adds 2 new functions for user to call:

/*
 * In single thread program, the function apiClient_setupGlobalEnv is not needed.
 * But in multi-thread program, apiClient_setupGlobalEnv must be called before any worker thread is created
 */
void apiClient_setupGlobalEnv()
{
    curl_global_init(CURL_GLOBAL_ALL);
}

/*
 * This function apiClient_unsetupGlobalEnv must be called whether single or multiple program.
 * In multi-thread program, it is must be called after all worker threads end.
 */
void apiClient_unsetupGlobalEnv()
{
    curl_global_cleanup();
}

Then user can create more than one api clients with multiple threads safely.

The difference compared with the original use case in single thread environment is:
User needs call the function apiClient_unsetupGlobalEnv at the end of program.

The detail discussion can be found at the issue: kubernetes-client/c#34

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[ityuhui]

Thanks @wing328