This commit actually does some different things:

  * When doing asynchronous decryption of incoming documents in soledad client
    during a sync, there was the possibility that a document corresponding to
    a newer generation would be decrypted and inserted in the local database
    before a document corresponding to an older generation. When this
    happened, the metadata about the target database (i.e. its locally-known
    generation) would be first updated to the newer generation, and then an
    attempt to insert a document corresponding to an older generation would
    cause the infamous InvalidGeneration error.

    To fix that we use the sync-index information that is contained in the
    sync stream to correctly find the insertable docs to be inserted in the
    local database, thus avoiding the problem described above.

  * Refactor the sync encrypt/decrypt pool to its own file.

  * Fix the use of twisted adbapi with multiprocessing.

Closes: #6757.

With changes in soledad client api to make it async, the supporting scripts
also have to be updated. This commit also adds more functionalities as
exporting incoming mail and public or private keys.