Add queue method to CBLDatabase to run Asynchronous workloads

[rismay]

Currently, you need to create a queue or a thread of your own to run CBL workloads in the background. CBL_Server has a queue method which allows you to perform asynchronous workloads. To solve this problem and keep CBL_Server private, add a queue method to CBLDatabase which then accesses it's private manager and server to queue the block of work.

[snej]

The danger of this is that it'd be really easy to misuse. The block has to use the CBL objects running on the background/server thread, not the ones being used by the main code. For example, this would be bad news:

[db dispatch_async: ^{[db compact];}

because it'd be calling db on the wrong thread. Instead you need something like:

[db dispatch_async: ^(CBLDatabase *bgdb) {[bgdb compact];}

but you only need to screw up once and use db instead of bgdb in the block to cause an error. (And it's an error that's likely not to cause any overt trouble most of the time, but occasionally crash or screw things up in mysterious ways.)

[snej]

Added a -tellDatabaseNamed:to: method to CBLManager in a commit today.

[rismay]

This solved my problem exactly. The CBL_Server code isn't hard to duplicate and I could have compiled a modified version from source. However, I think it's important to be able to run async workloads without having to spawn another thread or run a modified version of the official repository. Thanks!