[sync] Local transaction write client

[grigoryk]

Tolstoy needs an ability to insert a bunch of remote transactions as provided by the downloader (#511) into the local transactions table.

This includes:

• maintaining a txid->UUID mapping in tolstoy_tu
• updating the tx partition accordingly

This might also include performing raw assertions of datoms derived from the transactions (#512).

[rnewman]

The unstated assumption here: given a client at state A, including identifiers and datoms, where A was agreed with a server and (implicitly) other clients, then a tx T1 applied to A by another client should safely apply to the client without any changes.

[grigoryk]

Completed in #563.