Addressing issue #41 - maxSyncConcurrency environment variable

[smeyerhot]

Fixes #41

Motivation

This commit allows users to specify an env variable MAXSYNC which overrides DefaultConcurrency in rosetta-sdk-go.

Solution

The solution was to add a field to the Configuration struct in the same way as Mode and Network. Then used this value when constructing the syncer to override defaults.

Open questions

[smeyerhot]

This is a revised PR, same fix but cleared up some issues with the Dockerfile.