fix deadlock in bitswap sessions

[Stebalien]

This deadlock would happen when calling SessionsForBlock (holding bitswap.sessLk) while the session's main loop was trying to deregister the session (taking bitswap.sessLk).

I've also defensively added selects on contexts for two other channel writes just in case.

fixes #4394

...well, it fixes a deadlock showing up in that issue, there may be more.

[Stebalien]

Interesting error (unrelated, reported #4408):

make: *** [test/sharness/t0088-repo-stat-symlink.sh] Error 1
make: *** Waiting for unfinished jobs....
*** test/sharness/t0088-repo-stat-symlink.sh ***
# TEST_VERBOSE=1
# TEST_NO_FUSE=1
# TEST_NO_PLUGIN=
# TEST_EXPENSIVE=1
# TEST_OS=LINUX
expecting success: 
  mkdir sym_link_target &&
  ln -s sym_link_target .ipfs

ok 1 - create symbolic link for IPFS_PATH

expecting success: 
    export IPFS_PATH="$(pwd)/.ipfs" &&
    ipfs init --profile=test -b=1024 > /dev/null
  
ok 2 - ipfs init succeeds

expecting success: 
    mkdir mountdir ipfs ipns &&
    test_config_set Mounts.IPFS "$(pwd)/ipfs" &&
    test_config_set Mounts.IPNS "$(pwd)/ipns" ||
    test_fsh cat "\"$IPFS_PATH/config\""
  
ipfs config  "Mounts.IPFS" "/go/src/github.com/ipfs/go-ipfs/test/sharness/trash directory.t0088-repo-stat-symlink.sh/ipfs"
ipfs config  "Mounts.IPNS" "/go/src/github.com/ipfs/go-ipfs/test/sharness/trash directory.t0088-repo-stat-symlink.sh/ipns"
ok 3 - prepare config -- mounting

expecting success: 
  export IPFS_PATH="sym_link_target" &&
  reposize_direct=$(ipfs repo stat | grep RepoSize | awk '{ print $2 }') &&
  export IPFS_PATH=".ipfs" &&
  reposize_symlink=$(ipfs repo stat | grep RepoSize | awk '{ print $2 }') &&
  test $reposize_symlink -ge $reposize_direct

not ok 4 - 'ipfs repo stat' RepoSize is correct with sym link
#	
#	  export IPFS_PATH="sym_link_target" &&
#	  reposize_direct=$(ipfs repo stat | grep RepoSize | awk '{ print $2 }') &&
#	  export IPFS_PATH=".ipfs" &&
#	  reposize_symlink=$(ipfs repo stat | grep RepoSize | awk '{ print $2 }') &&
#	  test $reposize_symlink -ge $reposize_direct
#	

# failed 1 among 4 test(s)
1..4
test/sharness/Rules.mk:31: recipe for target 'test/sharness/t0088-repo-stat-symlink.sh' failed