Separate dynein configuration from table schema cache

[thash]

Issue #, if available: N/A

Description of changes:

Before this PR dynein saves detail of the currently using table in ~/.dynein/config.yml file.
Now, config.yml only contains using_region and using_table to identify the target table, and a new file named cache.yml would store details of tables you have used so far.

$ cat ~/.dynein/config.yml
---
using_region: ap-northeast-1
using_table: books

$ cat ~/.dynein/cache.yml
---
tables:
  us-east-1/Employee:
    region: us-east-1
    name: Employee
    pk:
      name: LoginAlias
      kind: S
    sk: ~
    indexes: ~
    mode: OnDemand
  ap-northeast-1/books:
    region: ap-northeast-1
    name: books
    pk:
      name: isbn
      kind: S
    sk: ~
    indexes: ~
    mode: Provisioned

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