Make region-client map concurrency safe

[lavafroth]

Changes

• A custom client map data structure is created as a sub-package under providers
• The client map structure uses a mutex for synchronization
• All concurrency unsafe map[string]*s3.Client instances are replaced with the custom structure
• The structure can be created using clientmap.New() or clientmap.WithCapacity(int)
• Key-value pairs can be retrieved or set using the Get and Set methods respectively
• Bare range over the structure is unavailable; hence we use a for i := ... loop over the length using the Len() method

[sa7mon]

@lavafroth This looks great. Thanks for the contribution!