Add support for ACTIVATE_VERSION message types

From the singer-python library:

ACTIVATE_VERSION message (EXPERIMENTAL).

The ACTIVATE_VERSION messages has these fields:
  * stream - The name of the stream.
  * version - The version number to activate.

This is a signal to the Target that it should delete all previously
seen data and replace it with all the RECORDs it has seen where the
record's version matches this version number.

Note that this feature is experimental. Most Taps and Targets should
not need to use the "version" field of "RECORD" messages or the
"ACTIVATE_VERSION" message at all.

msg = singer.ActivateVersionMessage(
    stream='users',
    version=2)

Implementing for taps:

I think we can safely implement for taps and send the message by default. For cases where targets cannot tolerate the unknown message types, we should support a disable_activate_version_messages=True option.

When FULL_TABLE replication is selected in the tap:

1. Initialize a version number (likely an epoch-based integer): https://github.com/transferwise/pipelinewise-tap-snowflake/blob/aa89f2e4235999dbeafc7406a7f8b382542d8d5b/tap_snowflake/sync_strategies/common.py#L33
2. Include version as property within emitted RECORD messages. https://github.com/transferwise/pipelinewise-tap-snowflake/blob/aa89f2e4235999dbeafc7406a7f8b382542d8d5b/tap_snowflake/sync_strategies/common.py#L200
3. Emit ACTIVATE_VERSION at the beginning of the first FULL_TABLE sync operation: https://github.com/transferwise/pipelinewise-tap-snowflake/blob/aa89f2e4235999dbeafc7406a7f8b382542d8d5b/tap_snowflake/sync_strategies/full_table.py#L87-L95
4. Emit ACTIVATE_VERSION after a successful FULL_TABLE sync: https://github.com/transferwise/pipelinewise-tap-snowflake/blob/aa89f2e4235999dbeafc7406a7f8b382542d8d5b/tap_snowflake/sync_strategies/full_table.py#L114