[main] Introduced asynchronous session opening
[main] Introduced bunch of QueryEndpoint.ExecuteAsync() methods
[main] Add .AsAsyncTask() extensions for running queries asynchronously
[postgresql] Fixed extraction of full-text indexes for certain scenarios
[postgresql] Set upper limit for precision of decimals to 49 to handle problems in Npgsql
[postgresql] Fields of unsigned types no longer cause exception on saving them to database
[postgresql] Enum fields no longer cause exception on saving them to database