Reuse sequelize instance across lambda invocations to improve performance

[prestonlimlianjie]

In order to fix a sporadic Sequelize connection error bug, we made a quick fix that re-initializes a Sequelize instance with each lambda invocation.

However, we found this new Sequelize documentation PR (still under review) that had recommendations on lambda best practices. A great recommendation is to:

• create a global sequelize variable (during the lambda Init phase, i.e. outside the handler)
• during the lambda Invoke phase (within the handler), restart connection pool using sequelize.connectionManager.initPools() and close the connections at the end of the phase using sequelize.connectionManager.close()

Refer to https://github.com/sequelize/sequelize/blob/114c10eb09353cd32a630e42454a5d587ab0aed3/docs/manual/other-topics/aws-lambda.md