Use new connection string for constructor

This removes the classical 'bucket', 'host', 'port', etc. options in
favor of the new standardized connection string.

Change-Id: Ia98084f03834b27ab5033f427b18c2e48a6ccaf6
Don't try to destroy instance on async connection if non exists

Change-Id: Ic962fa0e913062b47fe9e9926df6eabe7949c029
Use lcb_destroy_async for asynchronous destruction

Change-Id: I733b78a9fae837a5ae0bb7926ff2f28828214bac
Rename internal pycbc_Connection to pycbc_Bucket

Change-Id: Ib58214cc68f52d8eeed26127306cd2aa5148da4b
Move lcb_version() to module level

This is not a bucket-level property

Change-Id: I71ce20a5cb1ff4f81188addd90c71feae4bd502b
Rename Connection to Bucket

Note that there are still likely lots of doc references pointing to the
older name. This commit also includes a dummy subclass of Bucket called
Connection so that existing code will not break.

Change-Id: I22dc8564d9d009567a7e940c4e981a82372f6c6c