fix/ issue #164: Race condition in init() of connector classes

[OjusWiZard]

Before submitting this PR, please make sure:

• Your code builds clean without any errors or warnings
• You are using the approved title ("feat/", "fix/", "docs/", "refactor/")

A description of the changes proposed in the pull request:
This PR attempts to fix the issue #164. I observed that the cause of the errors in this issue is the following sequence:

1. The first /amm/price request is made.
2. This calls the init() function for the first time and the promise is returned which is awaited.
3. More subsequent /amm/price calls are made.
4. The ready() function still returns false because the first init() call is not finished yet.
5. So, all the subsequent /amm/price request again calls the init() function.
6. These latter calls skipped the initialization because this._initializing was true. Hence, these init() calls finished before the first init() call.
7. So, the latter price queries proceed as usual and fail at the getTokenBySymbol() because the tokens are yet to load from the first init() call.

Solution: The latter init() calls are made to wait while some other init() call is in progress. This is done by having a promise (this._initialized), that is unresolved whenever the init() is in progress.

Tests performed by the developer:
I tested the cases described in the issue thread around ten times. The errors didn't occur in any case.

[nikspz]

Test performed:

• Cloned and installed fix branch PR197 + latest development
• Connected uniswap arbitrum and created amm_arb using it
• started strategy, review bot showed profitability rate
• stopped gateway, restarted Client and gateway and review strategy logs showing no errors
• Start the strategy again and review issue was reproduced when gateway stopped and turn OFF
• start gateway again with yarn start —passphrase=password. Gateway turned into ONLINE again and recovered
• connected dexalot/pancakeswap and checked balance successfully after the restart
• created/started pancakeswap amm_arb successfully
• amm_arb order execution using uniswap arbitrum one after gateway restart: ok
• manually built docker image successfully
• Reviewed HB gateway compose was able to recover after gateway disconnected

[fengtality]

LGTM