Add a lock on a transport

[gre]

please review and test carefully.
this intend to fixes #83 , race condition when calling multiple times methods, like for instance signing 2 transactions in parallel.

this also fixes #97 , allowing multiple apps possible to be instanciated on the same transport.

when you try to call something that is "lock protected" when another thing is running (let's say signMessage is running), it will throw an exception "Ledger Device is busy (lock signMessage)".

@sgehrman would you mind checking if this fixed it for you? I have released version 4.3.0-beta.d226ed3b of each individual library on NPM that includes this PR changes.

Successfully published:
 - @ledgerhq/common@4.3.0-beta.d226ed3b
 - create-dapp@4.3.0-beta.d226ed3b
 - @ledgerhq/currencies@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-app-btc@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-app-eth@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-app-str@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-app-xrp@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-hid-cli@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-http-proxy-devserver@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-transport-http@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-transport-mocker@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-transport-node-hid@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-transport-u2f@4.3.0-beta.d226ed3b
 - @ledgerhq/hw-transport@4.3.0-beta.d226ed3b
 - @ledgerhq/react-native-hid@4.3.0-beta.d226ed3b
 - @ledgerhq/react-native-hw-transport-ble@4.3.0-beta.d226ed3b
 - @ledgerhq/web3-subprovider@4.3.0-beta.d226ed3b

thanks