Add concurrent HashMap for storing indexNames and connection objects

[rohanshah18]

Problem

Everytime createIndexConnection() or createAsyncConnection() is called, we are internally calling describeIndex() to get index host which is required for creating blocking or async stubs.

Solution

Create a singleton ConcurrentHashMap of indexName (key) and PineconeConnection (value) such that every time the PineconeConnection object is instantiated, the indexName along with the corresponding PineconeConnection is stored into the HashMap. The entry is removed when the connection object is closed.
Also for readability reasons, @ssmith-pc suggested to update the createIndexConnection() and createAsyncIndexConnection() methods to getIndexConnection() and getAsyncConnection() methods.

Type of Change

• New feature (non-breaking change which adds functionality)

Test Plan

Added unit tests and ran integration tests.

[ssmith-pc]

Also for readability reasons, @ssmith-pc suggested to update the createIndexConnection() and createAsyncIndexConnection() methods to getIndexConnection() and getAsyncConnection() methods

FWIW, I was also the one that suggested you rename them from getIndexConnection to createIndexConnection. Sorry!

[ssmith-pc]

Looking good, added a few comments.

At some point we'll need to add clear guidance on the proper (safe) usage especially in a multi-threaded context.

[aulorbe]

Some Qs just for my understanding and nits