When gRPC buffer of `Submit` stream is full, `SendSubmit` would
block, which freezes the `serveRequest` go routine. This CR moves
this out of go routine, and clients should be blocked on waiting
for room in buffer.

Change-Id: I62cd261b9419bd8df3fa1bfaeff14551168d2e65
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>