how to check connection

[ehooi]

Initially, I tried await client.checkHealth().
However, I couldn't catch any errors.
It caused the process to crash due to an unhandled promise rejection.

This happened because it does not await this.connectPromise, whereas checkCompatibility() does.

I believe there are currently two ways to check connection:

1. await client.connectPromise
2. await client.checkCompatibility()

However, I think await client.checkHealth() is the most intuitive way.

Therefore, I propose the following.
a. add documentation on how to check connection.
b. modify checkHealth() to await connectPromise internally.
c. change the API to explicitly establish a connection, for example: await client.connect()