async support

[sugizo]

tested pydal with blacksheep (add async on def function and await on db IO operation), but got an error
exposed/install.py

@app.route('/install', methods = ['GET'] )
async def install(request):
    if db(db.test).isempty():
        test_id_1 = await db.test.insert(name = "test1")
        test_id_1 = await db.test.insert(name = "test2")
    db.commit()
    return responses.redirect('/api/get/test/1')

result

Internal Server Error.
While handling request: GET /install

Exception type:
TypeError
Exception message:
object Reference can't be used in 'await' expression
Stack trace:

Traceback (most recent call last):
  File "blacksheep/baseapp.pyx", line 72, in blacksheep.baseapp.BaseApplication.handle
  File "/Users/sugizo/Downloads/blacksheep_pydal/test/exposed/install.py", line 10, in install
    test_id_1 = await db.test.insert(name = "test1")
TypeError: object Reference can't be used in 'await' expression
Process ID: 1885.

exposed/api.py

@app.route('/api/get/{tablename}/{rec_id}', methods = ['GET'] )
async def get(request, tablename, rec_id):
    query = (db[tablename]['id'] == rec_id)
    rows = await db(query).select().as_json()
    return responses.text(rows)

result

Internal Server Error.
While handling request: GET /api/get/test/1

Exception type:
TypeError
Exception message:
object str can't be used in 'await' expression
Stack trace:

Traceback (most recent call last):
  File "blacksheep/baseapp.pyx", line 72, in blacksheep.baseapp.BaseApplication.handle
  File "/Users/sugizo/miniconda3/envs/python3_test/lib/python3.8/site-packages/blacksheep/server/normalization.py", line 438, in handler
    return await method(*values)
  File "/Users/sugizo/Downloads/blacksheep_pydal/test/exposed/api.py", line 18, in get
    rows = await db(query).select().as_json()
TypeError: object str can't be used in 'await' expression
Process ID: 1916.

seems pydal not support async at the moment
it'll nice to have pydal support async

thanks